Fig. 74.—Army-worms, larvæ of the moth, Leucania unipuncta, on corn. (Photograph by M. V. Slingerland.)
Fig. 75.—The quince-curculio (a beetle),
Conotrachelus cratægi, natural size and
enlarged. (Photograph by M. V.
Slingerland.)
The June-beetles (June-bugs) (Lachnosterna sp.) feed on the foliage of trees. Their eggs are laid among the roots of grass in little hollow balls of earth, and the fat sluggish white larvæ feed on the grass-roots. They sometimes occur in such numbers as to injure seriously lawns and meadows. The larvæ live three years (probably) before pupating. They pupate underground in an earthen cell, from which the adult beetle crawls out and flies up to the tree-tops.
Hymenoptera: the ichneumon flies, ants, wasps, and bees.—Technical Note.—Obtain specimens of wasps, both social (distinguished by having each wing folded longitudinally) and solitary (wings not folded longitudinally), and if possible of both queens (larger) and workers (smaller) of the social kinds; of ants both winged (males or females) and wingless (workers) individuals; also of honey-bees, including a queen, drones, and workers, and some brood comb containing eggs, larvæ, and pupæ. The bee specimens can be got of a bee raiser. Compare the external structure of ants, bees, and wasps with that of other insects; note the pronounced division of the body into three regions (head, thorax, abdomen); note the character of the mouth-parts having mandibles fitted for biting (ants and wasps) or moulding wax (honey-bees) and having the other parts adapted for taking both solid and liquid food; note the sting (possessed by the females and workers only). Observe the behavior of bees in and about a hive; note the coming and going of workers for food. Observe bees collecting pollen at flowers; observe them drinking nectar. Examine the honey-bee in its various stages, egg, larva, pupa, adult. Note the special structure of the adult worker fitting it to perform its various special labors; the pollen-baskets on the hind legs; the wax-plates on the ventral surface of the abdomen, the wax-shears between tibia and tarsus of hind legs; the antennæ-cleaners on the fore legs; the hooks on front margin of hind wings, etc.
Fig. 76.—Immature stages of the quince curculio, Conotrachelus cratægi; at the left, the larva natural size and enlarged; at the right, the pupa. The beetle lays its eggs in pits on quinces, and the larva lives inside the quince as a grub; the pupa lives in the ground. (Photograph by M. V. Slingerland.)
The Hymenoptera include the familiar ants, bees, and wasps, and also a host of other four-winged, mostly small, insects, many of which are parasites in their larval stage on other insects. All Hymenoptera have a complete metamorphosis, and their habits and instincts are, as a rule, very highly specialized. The parasitic Hymenoptera such as the ichneumon flies, chalcid flies, etc., are stingless but have usually a piercing ovipositor (the sting being only a modified ovipositor). The general life-history of these ichneumons is as follows: the female ichneumon fly, finding one of the caterpillars or fly or beetle larvæ which is its host, settles on it and either lays an egg or several eggs on it, or thrusting in its ovipositor, lays the eggs in the body; the young ichneumon hatching as a grub burrows into the body of its caterpillar host, feeding on the body-tissues, but not attacking the heart or nervous system, so that the host is not soon killed; the ichneumon pupates either inside the host, or crawls out and, spinning a little silken cocoon (fig. 160), pupates on the surface of the body or elsewhere.
Some of the stingless Hymenoptera are not parasites, but are gall-producers. The female with its piercing ovipositor lays an egg in the soft tissue of a leaf or stem, and after the larva hatches the gall rapidly forms. The larval insect lies in the plant-tissue, having for food the sap which comes to the rapidly growing gall. It pupates in the gall, and when adult eats its way out.
Fig. 78.—The plum curculio,
Conotrachelus nenuphar, a
beetle very injurious to plums.
(Photograph by M. V. Slingerland.)
The ants, bees, and wasps are called the stinging Hymenoptera, although the ants we have in North America have their sting so reduced as to be no longer usable. Among these Hymenoptera are the social or communal insects, viz., all the ants, the bumblebees and honey-bee, and the few social wasps, as the yellow-jacket and black hornet. There are many more species of non-social or solitary bees and wasps than social ones, and their habits and instincts are nearly as remarkable.
Fig. 79.—The currant-stem girdler, Janus integer, a Hymenopteron at work girdling a stem after having deposited an egg in the stem half an inch lower down. (Photograph by M. V. Slingerland.)
The solitary and digger wasps do not live in communities as the hornets do, but each female makes a nest or several nests of her own, lays eggs and provides for her own young. The nest is usually a short vertical or inclined burrow in the ground, with the bottom enlarged to form a cell or chamber. In this chamber a single egg is laid, and some insects or spiders, captured and so stung by the wasps as to be paralyzed but not killed, are put in for food. The nest is then closed up by the female, and the larva hatching from the egg feeds on the enclosed helpless insects until full grown, when it pupates in the cell and the issuing adult gnaws and pushes its way out of the ground. Each species of wasp has habits peculiar to itself, making always the same kind of nest, and providing always the same kind of food. Some of these wasps make their nests in twigs of various plants, especially those with pithy centres in the stems. For interesting accounts of the habits of several digger wasps see Peckham's "The Solitary Wasps."
The solitary bees, of which there are similarly many kinds, are like the solitary wasps in general habit, only they provision the nest with a mixture of pollen and nectar got from flowers instead of with stung insects. Sometimes many individuals of a single species of solitary bee will make their nests near together and thus form a sort of community in which, however, each member has its own nest and rears its own young. In the case of certain small mining bees of the genus Halictus, a step farther toward true communal life is taken by the common building and use by several females of a single vertical tunnel or burrow from which each female makes an individual lateral tunnel, at the end of which is a brood-chamber. Perhaps half a dozen females will thus live together, each independent except for the common use of the vertical tunnel and exit.
The bumblebees (Bombus sp.) are truly communal in habit. All the eggs are laid by a queen or fertile female, which is the only member of the colony to live through the winter. In the spring she finds a deserted mouse's nest or other hole in the ground, gathers a mass of pollen and lays some eggs on it. The larvæ, hatching, feed on the pollen, dig out irregular cells for themselves in it, pupate, and soon issue as workers, or infertile females. These workers gather more pollen, the queen lays more eggs, and several successive broods of workers are produced. Finally late in the summer a brood containing males (drones) and fertile females (queens) is produced, mating takes place, and then before winter all the workers and drones and some of the queens die, leaving a few fertilized queens to hibernate and establish new communities in the spring.
The yellow-jackets and hornets (Vespidæ), the so-called social wasps, have a life-history very like that of the bumblebees. The communities of the social wasps are larger and their nests are often made above ground, being composed of several combs one above the other and all enclosed in a many-layered covering sac open only by a small hole at the bottom. This kind of nest hangs from the branch of a tree and is built of wasp-paper, which is a pulp made from bits of old wood chewed by the workers. The brood-cells are provisioned with killed and chewed insects, the larvæ of both solitary and social wasps being given animal food, while the larvæ of both solitary and social bees are fed flower-pollen and honey. As in the bumblebees, all the members of the community except a few fertilized females die in the autumn, the surviving queens founding new colonies in the spring. The queen builds a miniature "hornet's nest" in the spring, lays an egg in each cell and stores the cells with chewed insects. The first brood is composed of workers, which enlarge the nest, get more food, and relieve the queen of all labor except that of egg-laying. More broods of workers follow until the fall brood of males and females appears, after which the original process is repeated.
The honey-bees and ants show a highly specialized communal life, with a well-marked division of labor and an individual sacrifice of independence and personal advantage which is remarkable. Their communities are large, including thousands of individuals, and the structural differences among the males, females, and workers are readily recognizable. With the ants the workers may be of two or more sorts, a distinction into large and small workers or worker majors and worker minors being not uncommon.
A honey-bee community, living in hollow tree or hive, includes a queen or fertile female, a few hundred drones or fertile males, and ten to forty thousand workers, infertile females (fig. 80). The number of drones and workers varies, being smallest in winter. Each kind of individual has a certain particular part of the work of the whole community to do; the queen lays all the eggs, that is, is the mother of the entire community; the drones act simply as the royal consorts, fertilizing the eggs; while the workers build the comb, produce the wax from which the cells are constructed, bring in all the food consisting of flower-pollen and nectar, care for the young bees, fight off intruders, and in fact perform all the many labors and industries of the community except those of reproduction. There is a certain not very well understood and perhaps not very sharply defined division of these labors among the worker individuals, the younger ones acting specially as "nurses," feeding and caring for the young bees (larvæ and pupæ), the older ones making the food-gathering expeditions. The queen lays her eggs one in each of many cells (fig. 81). These eggs hatch in three days, and the young bee appears as a white, soft, footless, helpless grub or larva that is fed at first by the nurses with a highly nutritious substance called bee-jelly which the nurses make in their stomachs and regurgitate for the larva. After two or three days of this feeding the larvæ are fed pollen and honey. After a few days a small mass of this food is put into the cell, which is then "capped" or covered with wax. The larva after using up this food-supply pupates, and lies quiescent in the pupal stage for thirteen days, when the fully developed bee issues, and breaking through the wax cap of the cell is ready for the labors which are immediately assigned it. The bee with the kind of life-history just described is a worker. It has been demonstrated that the eggs which produce workers and those which produce queens do not differ, but if the workers desire to have a queen produced they tear down two or three cells around some one cell, enlarging this latter into a large vase-shaped cell. When the larva hatches from the egg in this cell it is fed for its whole larval life with bee-jelly. From the pupa into which this larva transforms issues not a worker but a new queen. The eggs which produce drones or males differ from those which produce queens and workers in being unfertilized, the queen having the power to lay either fertilized or unfertilized eggs. When a new queen appears or when several appear at once there is great excitement in the community. If several appear they fight among themselves until only one survives. It is said that a queen never uses its sting except against another queen. The old queen now leaves the hive accompanied by many of the workers. She and her followers fly away together, finally alighting on some tree-branch and massing there in a dense swarm. This is the familiar act of "swarming." Scouts leave the swarm to find a new home, to which they finally conduct the whole swarm. Thus is founded a new colony. "This swarming of the honey-bee is essential to the continued existence of the species; for in social insects it is as necessary that the colonies be multiplied as it is that there should be a reproduction of individuals. Otherwise as the colonies were destroyed the species would become extinct. With the social wasps and with the bumblebees the old queen and the young ones remain together peacefully in the nest; but at the close of the season the nest is abandoned by all as an unfit place for passing the winter, and in the following spring each young queen founds a new colony. Thus there is a tendency towards a great multiplication of colonies. But with the honey-bee the habit of storing food for winter, and the nature of the habitations of these insects, render it possible for the colonies to exist indefinitely, and thus if the old and young queens remained together peacefully there would be no multiplication of colonies and the species would surely die out in time. We see, therefore, that what appears to be merely jealousy on the part of the queen honey-bee is an instinct necessary to the continuance of the species."
Fig. 81.—Worker brood and queen cells of honey-bee; beginning at the right end of upper row of cells and going to the left is a series of egg, young larvæ, old larvæ, pupa, and adult ready to issue; the large curving cells below are queen cells. (From Benton.)
For the special labors of gathering food, making wax, building cells, etc., the workers are provided with special structures, as the pollen-baskets on the outer surface of the widened tibia of the hind legs, the wax-shears between the tibia and first tarsal joint of the hind legs, the wax-plates on the ventral surface of the abdomen, etc. A great many interesting things connected with the life and industries of a honey-bee community can be learned by the student from observation, using for a guide some book such as Cowan's "Natural History of the Honey-bee."
Fig. 83.—Comb of the tiny East Indian honey-bee, Apis florea, one-third natural size. (From Benton.)
The gathering of food from long distances, the details of wax-making and comb-building, of honey-making (for the nectar of flowers is made into honey by an interesting process), the storing of food, how the community protects itself from starvation when winter sets in or food is scarce by killing the useless drones and the immature bees in egg and larval stage, and many other phenomena of the life of the bee community present good opportunities for careful observation and field study. Although the community is a persistent or continuous one, the individuals do not live long, the workers hatched in the spring usually not more than two or three months, and those hatched in the fall not more than six or eight months. But new ones are hatching while the old ones are dying and the community as a whole always persists. A queen may live several years, perhaps as many as five. She lays about one million eggs a year.
There are more than two thousand known species of ants (fig. 84), all of which live in communities and show a truly communal life. The ant workers are specially distinguished in structure from the males and females by being wingless, and in numerous species there are two sizes or kinds of workers known as worker majors and worker minors. The life-history and communal habits of ants are not so thoroughly known as are those of the honey-bee, but they show even more remarkable specializations. The ant nest or formicary is with most species an elaborate system of underground galleries and chambers, special rooms being used exclusively for certain special purposes, as nurse-rooms, food-storage rooms, etc. The food of ants comprises many animal and vegetable substances, but the favorite food with many species is the "honey-dew" secreted by the plant-lice (Aphididæ) and scale insects (Coccidæ). To obtain this food an ant strokes one of the aphids with its antennæ, when the fluid is excreted by the insect and drunk by the ant. In order to have a certain supply of this food some species of ants care for and defend these defenseless aphids, which have been called the "cattle" of the ants. In some cases they are even taken into the ants' nests and food provided for them. "In the Mississippi Valley a certain kind of plant-louse lives on the roots of corn. Its eggs are deposited in the ground in the autumn and hatch the following spring before the corn is planted. Now the common little brown ant (Lasius flavus) lives abundantly in the cornfields, and is especially fond of the honey secreted by the corn-root louse. So when the plant-lice hatch in the spring before there are corn-roots for them to feed on, the little brown ants with great solicitude carefully place the plant-lice on the roots of a certain kind of knot-weed which grows in the field and protect them until the corn germinates. Then the ants remove the plant-lice to the roots of the corn, their favorite food-plant. In the arid lands of New Mexico and Arizona the ants rear scale insects on the roots of cactus."
Fig. 84.—The little black ant, Monomorium minutum; a, female, b, female with wings, c, male, d, workers, e, pupa, f, larva, g, egg of worker, all enlarged. (From Marlatt.)
The ants are among the most warlike of insects. Battles between communities of different species are numerous, and the victorious community takes possession of the food-stores of the conquered. Some species of ants live wholly by war and robbery. In the case of the remarkable robber-ant (Eciton), found in tropical and subtropical regions, most of the workers are soldiers, and no longer do any work but fighting. The whole community lives exclusively by pillage. Some kinds of ants go even farther than mere robbery of food-stores: they make slaves of the conquered ants. There are numerous species of these slave-making ants. They attack a nest of another species and carry into their own nest the eggs and larvæ and pupæ of the conquered community, and when these come to maturity they act as slaves of the victors, collecting food, building additions to the nest, and caring for the young of the slave-makers.
As with the honey-bee the larval ants are helpless grubs and are cared for and fed by nurses. The so-called "ants' eggs," the little white oval masses which we often see being carried in the mouths of ants in and out of an ants' nest, are not eggs, but are the pupæ which are being brought out to enjoy the warmth and light of the sun or being taken back into the nest afterward.
There are in this country numerous species of ants showing much variety of habit and offering excellent opportunities for most interesting field observations. For an account of several of the common species see Comstock's "Manual of Insects," pp. 633-643. Ants may be readily kept in the schoolroom in an artificial nest or formicary and their life-history and habits closely watched. For full directions for making and keeping a simple and inexpensive formicary see Comstock's "Insect Life," pp. 278-281. For an interesting account of some of the habits of the social insects see Lubbock's "Ants, Bees, and Wasps."
Class Myriapoda: The Myriapods, or Centipeds and Millipeds.
Belonging to the branch Arthropoda, with the classes Crustacea and Insecta, are three other classes, of which one, the Onychophora, is represented by a single genus Peripatus (Fig. 85), of extremely interesting animals. However, as these animals are not found in the United States we cannot study them. The other two classes are the Myriapoda, including the centipeds and millipeds or thousand-legged worms, and the Arachnida, including the scorpions, spiders, mites, and ticks. All these animals are often spoken of as insects, but though related to them they are not true insects.
Technical Note.—From under stones or logs obtain specimens of millipeds, or thousand-legged worms (large blackish, cylindrical, worm-like animals with each body-segment back of the fourth bearing two pairs of jointed legs); also specimens of centipeds or hundred-legged worms (flattened, usually brownish or pale worm-like animals with the body-segments bearing only one pair of legs each) in the same places. Examine the external structure; note number of body-rings; division into body-regions; presence of antennæ; character and number of eyes; character of mouth-parts; character and arrangement of legs. In the centipeds the first pair of legs is modified to form a pair of poison-fangs. They appear to belong to the mouth-parts. The internal anatomy will be found to be, if examined, much like that of insects and can be studied from the account of the anatomy of the water-scavenger beetle and butterfly larva. Compare the Myriapods with the Hexapods or true insects. What are the points of resemblance? what are the points of difference?
The Myriapoda are land-animals breathing by means of tracheæ like the insects. In them the body-segments are nearly uniform in character with the exception of the head, which, as in the insects, bears the mouth-parts and antennæ. There is no grouping of the body-segments into regions except as the head is opposed to the rest of the body. (In a few myriapods there are indications of a division of the hind body into thorax and abdomen.) The presence of true legs on all the segments of the hinder region of the body and the lack of the three-region division of the body are the principal external structural characteristics which distinguish myriapods from insects. The internal anatomy corresponds in general character with that of insects.
The most familiar myriapods are the millipeds, and the lithobians and centipeds. The millipeds are cylindrical in shape, have two pairs of legs on most of the body-segments and are vegetable feeders, though some may feed on dead animal matter. The galley-worms (Julus) (fig. 86), large, blackish, cylindrical millipeds found under stones and logs and leaves and in loose soil, are familiar forms. They crawl slowly and when disturbed curl up and emit a malodorous fluid. They can easily be kept alive in shallow glass vessels with a layer of earth in the bottom, and their habits and life-history may thus be studied. They should be fed sliced apples, green leaves, grass, strawberries, fresh ears of corn, etc. They are not poisonous and may be handled with impunity. They lay their eggs in little spherical cells or nests in the ground. An English species of which the life-history has been studied lays from 60 to 100 eggs at a time. The eggs of this species hatch in about twelve days.
Fig. 87.—The skein centiped,
Scutigera forceps, natural
size, common in houses
and conservatories. (From
Marlatt.)
The lithobians and centipeds are flattened and have but a single pair of legs on each body-ring. They are predaceous in habit, catching and killing insects, snails, earthworms, etc. They can run rapidly, and have the first pair of legs modified into a pair of poison-claws, which are bent forward so as to lie near the mouth. The common "skein" centiped (Scutigera forceps) (fig. 87) is yellowish and has fifteen pairs of legs, long 40-segmented antennæ, and nine large and six smaller dorsal segmental plates. The true centipeds (Scolopendra) (fig. 88) have twenty-one to twenty-three body-rings, each with a pair of legs, and the antennæ have seventeen to twenty joints. They live in warm regions, some growing to be very large, as long as twelve inches or more. The "bite" or wound made by the poison-claws is fatal to insects and other small animals, their prey, and painful or even dangerous to man. The popular notion that a centiped "stings" with all of its feet is fallacious. It is recorded by Humboldt that centipeds are eaten by some of the South American Indians.
Class Arachnida: The Scorpions, Spiders, Mites, and Ticks.
Technical Note.—Obtain specimens of various spiders; the running or hunting spiders may be found on the ground, especially under stones and boards, the web-makers on their snares. Get also spiders' "cocoons" (egg-sacs). Examine the external structure of the spider; note the two body-regions; the number and character of legs; the absence of antennæ; the number and arrangement of the eyes (which are simple, not compound); the mouth-parts, especially the large mandibles; the spinnerets at the tip of the abdomen (examine a cut off spinneret under the microscope to see the spinning-tubes); note the breathing openings or spiracles on under side of abdomen. Obtain also a scorpion if possible, and some ticks and mites. Compare with the spiders and note that in the scorpion the body is plainly seen (especially in the abdomen) to be composed of segments. Note the extreme fusion of the segments and body-regions in the mites and ticks. The common red spider of hothouses and gardens is a mite; ticks may sometimes be found on dogs. Observe various kinds of spider-webs, and try to observe the process of web-making (this can be observed early in the morning or about dusk) by one of the orb-weaving garden-spiders. Live spiders can be kept in the schoolroom and their feeding habits and perhaps web-making habits observed.
The class Arachnida is composed of Arthropods whose body-segments are grouped into two regions, a cephalothorax bearing the mouth-parts, eyes, and legs, and an abdomen. The segments composing these two parts are so fused that, except in the scorpions, they are usually indistinguishable. There are no antennæ, the eyes are simple, the mouth-parts fitted for biting, and there are four pairs of legs. In their internal anatomy the arachnids show in some forms a peculiar modification of the respiratory organs, the tracheæ being flat and leaf-like and massed together in a few groups rather than being tubular and ramifying through the body.
The dorsal vessel or heart usually has a few blood-vessels or arteries running from it. This class is divided into three orders, the Arthrogastra, or scorpions, the Acarina, or mites and ticks, and the Araneina, or spiders.
The scorpions (fig. 89) have the posterior six segments of the abdomen much narrower than the seven anterior segments and forming a tail which bears at its tip a poison-fang or sting. This sting is used to kill prey, insects and other small animals. The tail can be darted forwards over the body to strike prey which has been previously seized by the large pincer-like maxillary palpi. Scorpions are common in warm regions, about twenty species being known in southern North America. Their sting though painful is not dangerous to man. The young are born alive and are carried about by the mother for some time after birth.
The mites (figs. 90 and 91) and ticks (fig. 92) are mostly small obscure animals, which live more or less parasitically. The common red spider of house-plants as well as the sugar- and cheese-mites, the dreaded itch-mite and the chigger are familiar examples of these degraded arachnids, and the wood-ticks, dog- and chicken-ticks are common examples of the larger bloodsucking forms. The body in both mites and ticks is very compact, the two body-regions, cephalothorax and abdomen, being closely fused.
Fig. 91.—Bird mite, species undetermined, from the gnome-owl, Glaucidium gnomus. (Photo-micrograph by Geo. O. Mitchell.)
The spiders have the abdomen distinctly set off from the cephalothorax. The eyes (fig. 93) vary in number and arrangement, the mandibles are large, each being composed of two parts, a basal hair-covered part, the falx, and a terminal smooth, shining, slender, sharp-pointed part, the fang, which is movably articulated with the falx (fig. 93). In the falx is a poison-sac from which poison flows through the hollow fang and out at its tip. The legs vary in relative length in different spiders, and each is made up of seven joints. The spinnerets (fig. 94), which are situated at the tip of the abdomen, are six in number (a few spiders have only four), and are like little short fingers. They have at their tips many fine little spinning-tubes from each of which a fine silken thread issues when the spider is spinning. These many fine threads fuse as they issue to form a single strong cable or sometimes a flat rather broad band. The spinnerets are movable, and by their manipulation the desired kind of line is produced. The silk comes from many silk-glands in the abdomen, from each of which a fine duct runs to a spinning-tube.
Fig. 92.—The dog or wood tick, Dermacentor americanus male, the most common tick in the Northern States. (After Osborn.)
The spiders may be divided into two groups according to their habits, viz., the wandering or hunting spiders, which do not spin webs to catch their prey, and the sedentary or web-weaving spiders, which spin snares to catch their prey. The wandering spiders can spin silk, however, and often do so to line their burrows, to make nests, or to make egg-sacs.
The hairy tarantulas and the trap-door spiders of similar appearance are among the most interesting of the hunting spiders. They live in vertical burrows or tunnels in the ground which are lined with silk, and which in the case of the trap-door spider are covered with a door or lid made of silk and soil. The top of this door is always covered with soil or bits of leaves or twigs so that it is nearly indistinguishable from the surface of the ground about it. When the nest is in ground covered with moss the spider covers the door with moss. The tarantulas hunt at night and rest in the burrow in the daytime. They are very large, sometimes having an expanse of legs of 6 inches.
Fig. 97.—A female running spider (Lycosidæ) carrying its egg-sac about attached to its spinnerets. (From Jenkins and Kellogg.)
The common, rather large swift black spiders found under stones and boards are hunting spiders, belonging to the family Lycosidæ and are called the running spiders (fig. 96). They live in burrows in the ground, coming out to stalk and chase their prey. The eggs are laid in globular egg-sacs which are often carried about, attached to the spinnerets, by the female (fig. 97). The young spiderlings after hatching, in some species, climb on to the mother's back and are carried by her for some time. Other kinds of wandering or hunting spiders are the crab-spiders (Thomisidæ) (fig. 98), which run sidewise or backward as well as forward, and the black and red, fierce-eyed stout-bodied little jumping spiders (Attidæ) (fig. 99), which leap on their prey.
The sedentary or web-weaving spiders are of various kinds. They may be grouped according to their spinning habits into cobweb weavers (Therididæ), small slim-legged spiders which make the familiar unsymmetrical cobwebs of houses and outbuildings; funnel-web weavers (Agalenidæ), larger long-legged spiders of meadow and field which spin a flat or concave horizontal web in the grass with a silken tube leading down to the ground; the curled-thread weavers (Dictynidæ), which use in addition to the usual lines peculiar broad lines made of waved or curled threads in their irregular webs made in fence-corners and on plants; and finally orb-weavers (Epeiridæ) (fig. 100), the host of variously colored and patterned stout-bodied garden-spiders which spin the beautiful symmetrical circular webs familiar to all (fig. 101). If a complete uninjured orb web be examined it will be found to consist of a small central hub either open or closed, from which run radii to the outer edges of the web. Around the hub is an open or free zone, and farther out a spiral zone, so called because a line running in close spiral turns fills in the space between the radii. This is the real prey-catching part of the snare, and the silken line here is sticky, while the radii and some other parts of the web are made of silk that is not sticky. The web is supported by strong foundation-lines, attached to leaves, stems, or whatever is firm in the neighborhood of the web. The spider either rests on the web, usually in the centre, or lies concealed in a nest or tent near at hand from which a special path-line runs to the centre of the web. The building of one of these orb webs is a great work, and is done with extraordinary nicety of manipulation by the use of feet and spinnerets. For account of web-making, etc., see McCook's "American Spiders and their Spinning Work."
Fig. 101.—Spider and its web in a rose-bush. (Photograph from life by Cherry Kearton; from "Wild Life at Home," by permission of Cassell & Co.).
The habits and instincts of spiders in connection with the care of the young, the building of webs and nests, ballooning by means of silken lines, the active stalking and catching of prey, etc., are very interesting and offer a good field for independent observation and study by the student.
Fig. 102.—The triangle spider, Hyptiotes sp. (California), with its web; the spider rests on the taut guy-line, with a loop of the line held between its fore and hind legs; when an insect gets into the web the spider loosens the hold of its hind feet on the guy-line, thus allowing the web to spring forward sharply and further entangle the prey. (From Jenkins and Kellogg.)
CHAPTER XXII
MOLLUSCA: THE MOLLUSCS
THE FRESH-WATER MUSSEL (Unio sp.)
Structure (fig. 103).—Technical Note.—The fresh-water or river mussel lives commonly in the streams and lakes or ponds in the United States. It frequents muddy or sandy bottoms. Specimens can often be secured with a long-handled rake from the shore or picked up in shallow streams with the hand. If possible to keep the animals alive until ready for use, some of their habits may be observed. Place them in a tub or trough with water and mud; when they have settled themselves put some powdered carmine, starch, or similar substance in the water near them, and note the water-currents.
Living mussels which have been placed in a dish with mud several inches deep and covered with water will be seen to travel in a definite direction. The end which is in front is the head end. Note the process of thrusting out and retracting the fleshy foot which extends between the two valves of the shell. Note that the two valves are held together along the upper, or dorsal, surface by a horny structure, the hinge-ligament. Note near the hinge-line a prominence (umbo) in each valve from which extends a series of concentric lines of growth. The umbo is the oldest part of the valve. Note at the lower edge of the valves a soft membrane with a fringe along its free border. This is the edge of the mantle-lobes, flaps of the body-wall which cover the body and which aid in the functions of respiration and nutrition.
Technical Note.—Specimens which are to be dissected should be killed by dropping them for a few seconds into warm water, when the muscles will relax enough so that a chip may be thrust between the valves. If specimens are to be kept for some time before dissecting they should be preserved in alcohol or 4% formalin. In a dead specimen carefully remove the left valve. This is accomplished by slipping in a thin knife-blade close to the inner edge of the left valve and carefully cutting the two large adductor muscles which bind the valves together. The dissection should be made under water.
Before the removal of the valve, as just described, notice a portion of the mantle adhering to the inner face of the valve, along a line of attachment indicated by a crease. This is the pallial line. After the left valve has been removed, the mantle being carefully separated from it, note the large conical projections from the valves, the hinge teeth, which fit into each other. Note the large muscle impression just in front of the hinge-teeth; this is the point of attachment of the anterior adductor muscle, while just behind and adjoining it is the impression of the anterior retractor muscle. Note posterior to the adductor and below the retractor a small impression which affords attachment for the protractor muscles of the foot. At the other end of the valve, note the large impression of the posterior adductor muscle with the impression of the small posterior retractor muscle just above it.
Technical Note.—Lift back the left mantle-lobe, thus exposing the body parts underneath.
Note the projecting muscular foot, the movements of which are governed by the retractor and protractor muscles attached to the impressions just mentioned. Note a pair of flattened plate-like structures composed of thin, ribbed, membranous folds. These are the gills. Note just beneath the anterior adductor muscle a small opening leading into the soft visceral mass of the body. This is the mouth. Note near the mouth two pairs of plate-like structures much smaller than the gills. These are the labial palpi, and it is by their action that food-particles which have been brought in with the water are conveyed to the mouth. Note at the posterior part of each mantle-lobe a fringed portion which, together with a corresponding part on the other side, forms the inhalant siphon. The cilia of the fringes carry water and food-particles into the space enclosed by the mantle-lobes; this space is the mantle-cavity. After the food has been taken out and the water has passed through the finely striated gills it is collected in a common cavity which extends above the two sets of gills on each side. This space is called the supra-branchial cavity. This cavity is continuous posteriorly with a space between the right and left mantle-lobes, which is connected with the exterior by an opening above the inhalant siphon called the exhalant siphon. The function of the gills is partly to produce currents of water carrying the food to the mouth, and partly respiratory. The mantle is an important organ of respiration.
Make a drawing showing the organs described.
Technical Note.—Carefully cut away the mantle and gills from the left side, and also the labial palpi, being careful not to disturb the visceral mass.
Note two openings along the line where the gills and foot come together. The uppermost is the opening of the ureter giving exit to the excretion from the kidneys; the lower is the opening of the duct from the reproductive organs and is called the genital aperture. The products from both of these organs are carried out through the exhalant siphon.
Note that the mouth leads by a short tube (œsophagus or gullet) into a large cavity, the stomach, which is surrounded by a greenish mass, the digestive gland.
Technical Note.—Carefully cut the delicate covering of the dorsal portion of the visceral mass and expose a cavity.
The cavity thus exposed is the pericardium. Note within the pericardium a long tube extending through it. This is a portion of the alimentary canal, the rectum, which opens posteriorly through the anus into the supra-branchial chamber. Note a muscular sac about the rectum midway of its course through the pericardium. This is the unpaired ventricle of the heart. Attached to each side of the ventricle are thin-walled sacs, the right and left auricles, which are entered by fine blood-vessels, the efferent branchial veins, from the right and left gills. The blood brought through these blood-vessels from the gills flows into the auricles and from them into the unpaired muscular ventricle, from which it is forced anteriorly and posteriorly through two main arteries, the anterior and posterior aortas, to all parts of the body. After bathing the body-tissues the blood is collected into a median longitudinal vein beneath the pericardium called the vena cava. From the vena cava the blood passes through the kidneys and gills to be returned at last to the heart. The mantle acts as an organ for the aeration of the blood, and the blood it receives or at least part of it passes directly back to the heart without passing through the kidneys and gills.
Note the delicate membranous dark-colored sac on the floor of the pericardium, the kidneys or nephridia. These are paired structures which appear as two U-shaped tubes lying side by side. Each consists of a lower portion with thick folded walls, the kidney proper, and an upper thin-walled portion, the ureter. The kidneys open internally through a pair of reno-pericardial openings into the pericardium, while the ureters communicate with the mantle-cavity by an opening on the side of the body beneath the gills as already mentioned. The kidneys are profusely supplied with fine blood-vessels and carry off the waste matter from the blood.
Beneath the posterior adductor muscles note a small white spider-shaped body, the more or less united visceral ganglia of the nervous system. Posteriorly these ganglia give off nerves to the mantle and gills, while anteriorly there proceed two nerves, the cerebro-visceral connectives, running forward, one on either side of the foot close to the visceral mass, to the cerebro-pleural ganglia, paired ganglia lying near the mouth. A delicate commissure running over the gullet connects these ganglia.
Technical Note.—Cut away the skin and outer muscular layer from the left side of the foot.
Note the large stomach-cavity, surrounded by the digestive gland. Trace the convolutions of the alimentary canal through the foot to the anal exit. Note in the anterior portion of the foot a fused pair of ganglia similar to the visceral ganglia. These are the pedal ganglia, which are connected by a pair of delicate commissures, the cerebro-pedal connectives, with the cerebro-pleural ganglia. Note the glandular tissue which fills the cavity of the foot and surrounds the loops of the alimentary canal. This is the reproductive organ, which has its exit beneath the gills on each side of the foot. The sexes of the mussel are separate, but the reproductive organs are very similar.
Life-history and habits.—The eggs (ova) of the female pass first into the supra-branchial chamber, whence, after being fertilized, they drop into the outer pair of gill-chambers. These outer gills serve as brood-pouches, and here it is that the embryonic stages are passed through. The embryo when ready to issue has a soft body enclosed in two triangular valves. At this stage it is called a glochidium. The glochidium on being discharged through the exhalant siphon of the parent falls to the bottom, where it remains for a time, when it attaches itself to some fish by the lower hook-like projections of the valves and leads a truly parasitic life for two months, after which it undergoes a metamorphosis and falls to the bottom again, there to begin an independent existence. Mussels often congregate in favorite mud or sand banks. Their food consists primarily of small organisms, both plants and animals, which are taken from the water entering the mantle-cavity. Mussels move about slowly over the muddy bottom of the stream by means of the muscular foot.
OTHER MOLLUSCS.
The branch Mollusca includes the fresh-water mussels, the clams, oysters, snails, and slugs, the cuttlefishes, and all that host of animals we call "shells" or shell-fish, which we know familiarly only by the shell which they make, live in, and leave at death to tell the tale of their existence. Not all the molluscs, however, form shells, that is, external shells which serve as houses. The familiar slugs do not, nor do a number of ocean forms called nudibranchs, which are somewhat like the land-slugs, only much prettier and more attractive. All the cuttlefishes and octopi are also without the hard calcareous shell. But most of the molluscs are shell-bearing animals. The shell may be bivalved, as in the mussel and clam, or univalved, that is, composed of a single piece which may be spirally twisted, as with the snail, or otherwise curiously shaped. The variety in the form, colors, and markings of the shells indicates the great diversity among molluscs. Molluscs live on land, in fresh water and in the ocean. No depths of the ocean abysses are too great for the octopi, no coast but has its many shells, hardly a pond or stream is without its mussels and pond-snails, and in all regions the land-snails and slugs abound.
Body form and structure.—The molluscs are not to be mistaken for any other of the lower animals; they have a structure peculiarly their own. In them the body is not articulated or segmented as with the worms and arthropods, nor radiate as in the echinoderms, nor plant-like as with the sponges and polyps. (Where the typical molluscan body is well developed it is composed of four principal parts: a head, with the mouth, feelers, eyes, and other organs of special sense; a trunk containing the internal organs; a foot which is a thick muscular mass not at all foot- or leg-like in shape, but which is the organ of locomotion by means of which the mollusc crawls; and a mantle which is a fold of the skin enclosing most of the body and which produces the shell. Such a typical molluscan body is possessed by most of the snails. But in most of the other molluscs one or more of these four body-regions are so fused with some other region as to be indistinguishable. In the mussels and clams the head is not at all set off from the rest of the body, the cuttlefishes and octopi have no foot, the slugs have no shell. In the case of some of the molluscs without external shell there are inside the body the rudiments or vestiges of a shell.
With regard to the internal organs we note the constant presence of three pairs of ganglia, viz., the brain, lying above the pharynx, which sends nerves to the feelers, eyes, and auditory organs; the pedal ganglion, which sends nerves to the foot, and the visceral ganglion, which sends nerves to the viscera. This is a condition of the nervous system characteristic of all molluscs. The heart is a well-developed pulsating sac in the upper part of the body composed of either two or three chambers, and there is a well-defined closed system of arteries and veins, specially complete in the cuttlefishes and octopi. This highly developed condition of the circulatory system also distinguishes the molluscs from the other invertebrates.
Development.—Reproduction among the molluscs is always sexual. Multiplication by budding or by the parthenogenetic production of eggs is not known to occur. The eggs are usually laid in a mass held together by a gelatinous substance. In most species the young mollusc on hatching from the egg does not resemble its parent, but is a free-swimming larva called a veliger. It is provided with cilia for organs of locomotion. It must undergo a radical change in order to reach the adult stage. Thus metamorphosis occurs in this branch as well as among the Arthropods and Echinoderms. In the development of some molluscs, however, there is little or no metamorphosis, the young being hatched in a condition much resembling, except in size, the parent.
Some of the special characteristics of structure, life-history, and habits of the molluscs will be noted in our consideration of the various kinds.
Classification.—The branch Mollusca is divided into five classes, three of which include the more familiar kinds. These three classes are the Pelecypoda, including the mussels, cockles, clams, scallops, oysters, etc., molluscs with a shell composed of two pieces, one on each side of the body and hinged together; the Gastropoda, including the snails, slugs, periwinkles, whelks, and a host of other univalved shell-fish, that is, molluscs which have a shell composed of a single piece; and the Cephalopoda, including the squids, cuttlefishes, octopi, and the pearly nautilus.
Clams, scallops, and oysters (Pelecypoda).—Technical Note.—Shells of scallops, oysters, and sea-mussels should be had for examination; also specimens of Teredo or Pholas in alcohol or formalin, and pieces of pile bored by Teredo. Make drawings of various bivalve shells, and of Teredo.