How to Collect and Preserve Insects

8. Complete the assembly of the aspirator by placing the rubber stopper snugly into the vial. The vial aspirator is now ready for use.

If a piece of glass tubing 1¼ inches in diameter and 8 inches long is available, then a tube aspirator can be made, fig. 5. In making a tube aspirator, use two one-hole rubber stoppers, 5b. Cut the pieces of glass tubing as shown in fig. 5c, 5e, and 5g. Using a piece of rubber tubing, 5f, complete the minor details as described for the vial aspirator, not forgetting the cheesecloth, 5e, and assemble the parts as shown in fig. 5.

Use and Care.—To catch an insect with the aspirator shown in fig. 4 or fig. 5, put end piece, g, in your mouth, grasp the vial or tube, a, in your dexterous hand, aim the intake tube, c, at the insect and almost touching it; suck suddenly and hard. The air current pulls the insect in; the insect usually does not find its way into the intake tube to crawl out. The cheesecloth, e, prevents the insect from being sucked into your mouth.

To kill insects in the aspirator, use a small cyanide bottle, 4i, which is inserted in a cork, 4h, that has been partially bored through to receive it. This cork should be the exact size of the vial or tube for which it is intended.

To use the cyanide cork with the vial aspirator shown in fig. 4, simply exchange the corks. If the glass tube aspirator is used, plug the intake tube, 5c, with a tapered paper plug or a leaf, jar the insects away from the stopper at the opposite end, remove this stopper cautiously, and quickly insert in its place the cyanide cork. When the specimens are stupefied, they may be transferred to another bottle.

The cyanide corks are highly poisonous. Between times of use with an aspirator, keep each of these corks inserted tightly in a bottle or vial labeled “POISON,” as in fig. 3.

Sifter

Perhaps no special collecting method results in more interesting, rare, and diverse kinds of insects than that involved in sifting rotten logs, leaf mold, and other forest and prairie ground cover. To do this type of collecting, provide yourself with the following:

1. A stout sifting sieve about 12 by 12 inches and 4 to 6 inches deep, fig. 6. The bottom may be wire screen of any desired mesh; usually 8, 10, or 12 meshes to the inch give good results.

2. A sturdy piece of white oilcloth about 18 inches or 2 feet square.

3. Collecting equipment, including an aspirator, camel’s-hair brush, forceps, vials, and killing bottle.

Material such as leaf mold is placed in the sieve and this is shaken over the white oilcloth, which has been spread on a level spot on the ground. The small insects fall on the cloth and can be picked up with the aspirator or the camel’s-hair brush. Many insects feign death when they fall to the oilcloth and they are difficult to detect in the bits of sifted material until they “revive” and start to move.

In late fall and winter, sifting provides one of the most profitable types of collecting; in any season, it will turn up such things as rare spiders and beetles. Sifting is most successful for finding large, active insects. For small, slow-moving forms, Berlese funnels offer a better collecting method.

Berlese Funnel

When you are wandering through woods or fields, do you realize that you are stepping on more insects than you ever see? The ground cover and soil are inhabited by a vast assemblage of little animals that are seldom seen by the casual collector. Because many of these animals are exceedingly minute, they are difficult to see and collect by ordinary methods.

Construction.—The most efficient method for collecting this fauna is by the use of Berlese funnels, named after the Italian entomologist Berlese (pronounced Bur-lazy), who first used them extensively. A Berlese funnel is a very simple apparatus, consisting of a fairly long funnel, suspended wide end up, with, a screen placed about a third of the way down the funnel; heat is applied either around the upper portion or over the top of the funnel, and a container of preservative, preferably 80 per cent ethyl alcohol, is placed at the small bottom opening. Leaf mold or other material is placed on the screen, the heat source is turned on, and soon the animals begin to leave the drying sample and migrate downward, dropping into the preservative.

Fig. 7 illustrates a funnel that has proved very satisfactory; it is 15 inches from top to bottom, and the top has a diameter of 12 inches. The bottom opening, seven-eighths inch in diameter, fits into the mouth of the bottle containing the preservative. Three angled brackets or hangers are soldered inside the funnel to provide a rest for the screen, which is made of quarter-inch or eighth-inch mesh hardware cloth; the mesh used depends upon the type of sample. A battery of several funnels in a rack, fig. 8, will allow the collector to sample several kinds of material at the same time.

If steam is used as a source of heat, the small copper lines that conduct it act as a partial support for the funnel by encircling it about halfway between the screen and the top; a piece of cloth is tied tightly over the top of the funnel to prevent the upward escape of animals. If an electric light is used for heating, it should be hung directly over the center of the funnel, no cloth should be tied over the top, and the light should have a reflector nearly as wide as the top of the funnel.

Care must be taken not to heat the sample too rapidly. Otherwise, either moisture will condense in the lower part of the funnel and trap many of the animals working their way toward the bottom, or the heat will kill many of the organisms before they have an opportunity to move out of the sample. An application of heat sufficient to dry the sample in 4 or 5 days is usually satisfactory.

The Berlese funnel is extremely useful for collecting many groups of beetles (particularly Staphylinidae), thrips, springtails, many groups of parasitic Hymenoptera, ants, mites, pseudoscorpions, millipedes, centipedes, and a wide range of other minute animals that live in soil, surface cover, logs, or bark.

Collecting Berlese Samples.—Many different habitats and micro-habitats provide good samples for the Berlese funnel. You will find that, for general collecting, various types of ground cover are excellent; for leaf mold samples, scrape off and discard the dry surface leaves and scoop up the lower, rotted layers of leaves together with an inch or two of the adjacent soil. You may encounter especially good samples where leaves have blown in along the edge of a log. In such a situation, take some of the log bark with the sample. Collect rotten log samples in large hunks and break them up just before putting them in the funnel. From either standing stumps or fallen logs in which the wood is still too hard to break up, collect the loose bark, as it is often quite productive. Frequently, if you roll a log over, you may find animal runs under it; the debris and earth under and around these runs, together with animal nests, frequently give unusual catches, such as larvae and adults of fleas and rare ticks. Especially productive are samples taken from the interior of a standing hollow tree; from the bottom of the hollow you can scoop out a foot or more of fine, rotten, woody material rich in rare insects.

Certain items placed in the funnel may produce distinctive and unusual catches. Recently deserted birds’ nests will give mites and, frequently, rare beetles, flies, and their larvae; mature or overmature mushrooms and bracket fungi are often rich in beetles, thrips, and maggots; bark of living trees may produce unusual thrips, springtails, and psocids; debris from aquatic habitats and from the wet edges of ponds and tiny streams may be productive of rare aquatic and semiaquatic forms. Moss is a good source of peculiar species of springtails, thrips, and beetles; the moss should be rolled up carefully while being transported.

Handling Berlese Collections.—In the field, put samples of leaf mold or other material in tightly woven cloth bags or strong paper bags for transportation. It is convenient to have small paper bags for mushrooms, nests, and other small items, and larger bags for ground cover, moss, and the like. When collecting ground cover and similar material, put in each bag enough of a sample so that it will not shake around loosely, but do not pack it tightly. Be sure that samples do not overheat while being transported.

Samples may be collected at any season. If collected during warm weather, they should be taken to a laboratory and placed in the funnels within a day or two; otherwise, considerable loss of population occurs within the samples. If collected during cold weather, they may be kept in cold storage for a week or two with little loss of fauna.

In putting material in the funnel, lay it carefully on the screen to a depth of a few inches. Moss and sod should be placed upside down in a single layer on the screen. In the case of dense material, pile the sample chiefly around the sides of the funnel and leave an opening in the middle, as shown in fig. 7. After the funnel is loaded, place it in the rack, put the bottle of preservative under it, and apply the heat.

By substituting a different kind of collecting bottle at the bottom of the funnel, you may obtain live material for rearing. The exact changes necessary to obtain live material will depend upon your ingenuity and the type of material you desire.

Equipment for Collecting Aquatic Insects

Hundreds of different kinds of insects are aquatic and offer rich collecting possibilities. In all instances, the immature stage lives in water, but in most of them the adult stage emerges on land or flies in the air. For this reason several types of collecting are needed to obtain a good sampling of aquatic insects.

Night Collecting of Adult Insects.—Collecting at lights on warm, cloudy nights, or warm nights without moonlight, gives best results. Two simple methods are as follows:

Drive your car to a spot overlooking a stream or lake and turn on the bright lights. Into a shallow pan, such as a pie pan, pour enough alcohol to cover the bottom with one-eighth to one-fourth inch of fluid. Hold the pan directly under a headlight. If aquatic insects are on the wing, they will come to the light and eventually drop in the fluid, which traps them. With a small piece of wet cardboard, you can scrape the entire insect contents of the pan into a small bottle of alcohol, which you should then label, giving date, name of collector, and location.

Lights in signs and store windows (especially blue neon signs) near fresh water attract large numbers of aquatic insects. You may capture an insect easily by dipping an index finger in a bottle of alcohol, “scooping up” the insect rapidly on the wet surface, and then dipping it in the bottle. An aspirator also can be used with success.

Day Collecting of Adult Insects.—During the day, aquatic insects frequently rest on or under bridges, window ledges, and similar places, and show a preference for dense trees in shaded situations. They are especially numerous in those spots where the heavily leaved branches hang low over the water and form humid, protected areas in the heat of the day. Here sweeping with a stout and fairly wide-mouthed net is very effective. Aquatic insects may often be picked off stones in such places, especially early in the season.

Collecting Larvae.—Practically every stream or lake has some aquatic insect larvae which may be collected by various methods, some simple and others requiring specialized and complicated apparatus. For general collecting, the following suggestions may be of value:

1. Look under logs and stones. Search out crevices in them; some insects hide away and demand of the collector a keen and careful search.

2. Tear apart bunches of leaves, roots, and other debris that may have piled up in front of a rock or log, or that may have accumulated at the end of a root or branch dangling in the water.

3. Pick out bunches of aquatic plants and search through them carefully.

4. Sift mud, sand, or gravel taken from the bottom of a lake or stream. Remember that some insects build cases in which they hide when disturbed. It takes a practiced eye to see a motionless case. After an insect has dried out a little, it partially emerges from the case and drags it along in search of water; moving in this way, it is easy to see.

HOW TO HANDLE UNMOUNTED SPECIMENS

Soon after insects are killed they dry out, become very brittle, and are damaged easily. Small, fragile insects especially are susceptible to breakage and, when dry, break up readily into fragments. Hard-shelled insects, such as beetles, may appear to be sufficiently durable to withstand handling when dry, but even these insects have fragile legs, antennae, and other parts which snap off readily when handled dry. Newly killed material should be either mounted or put in temporary storage before it has dried out. If collected material dries out before it can be mounted or stored, it should be relaxed by special techniques so that the specimens can again be handled without danger of breakage.

Temporary Cases

If it is not convenient to mount the specimens when they are taken from the killing bottle, the moths and butterflies should be put in papers and other insects in cellucotton.

Papers are simply rectangular strips of paper of convenient size folded as in fig. 9. A moth or butterfly, with its wings folded, is placed in a paper, the edges of which are then crimped over to lock it shut.

For insects other than moths or butterflies, cardboard pillboxes containing cellucotton make good temporary housing, fig. 10. A layer of cellucotton is laid in the bottom, a layer of insects placed on it, and another layer of cellucotton placed over the insects. The lid should fit fairly snugly over all. Cigar boxes and other boxes of like size also may be used in the same way.

Great care must be taken that sufficient cellucotton is put in the box to take up all moisture in the insect bodies. If the specimens are large, they should be allowed to dry moderately uncovered before being placed in cellucotton in storage containers. If insects become damp in the containers they quickly mold or rot. The containers should be wood or cardboard boxes, for they will not sweat, as will a metal box. The insects should be packed tightly enough to prevent their rolling around and breaking.

Relaxing Boxes and Jars

At any desired time the dry specimens may be relaxed and mounted. A relaxing box or jar is easily made. In the bottom of a wide-mouthed jar with a screw-on lid, put an inch or two of clean sand; saturate the sand with water containing a small amount of phenol (carbolic acid) and place over it a piece of cork, cardboard, or wood cut to fit the jar. Place the dry specimens on the cork or other material, and cover the jar tightly with the screw-on lid. The lid must be practically airtight. In a day or two the specimens will be soft and pliable enough for pinning or spreading, the next steps toward permanent arrangement of the collection.

The relaxer will sweat if kept in too hot a room and will spoil the specimens. Also, the insects will be spoiled if left in the relaxer too long. The correct length of time varies with each relaxer and can be learned only by experience.

HOW TO MOUNT AND PRESERVE SPECIMENS

Most adult insects in collections are mounted on pins. Most medium-sized to large insects, such as grasshoppers, butterflies, moths, flies, bees, and many beetles should be pinned directly through the body from top to bottom. Many small insects, such as leafhoppers, plant bugs, small beetles, and the like, should be glued on card points. Immature insects and the adults of some groups are best preserved in fluid.

Preservation by Pinning

Hard-bodied insects, such as beetles, flies, and wasps, are preserved as dry specimens on pins better than in fluid. The pinned specimens are more convenient to study and they retain their natural coloring better. Flies and butterflies are covered with hairs or scales that clot or break off if the specimens are preserved in fluid, and for this reason they should be pinned.

Common household pins are too thick and short for pinning insects. Longer, slender pins called insect pins are necessary and may be purchased from various supply houses. They should be of spring steel; a brass pin will corrode and be destroyed by acids in the insect’s body. The pins are available in numbered sizes, of which 1, 2, 3, and 4 will be found of most general use, and sizes 0 and 00 of advantage in special cases.

Medium to Large Insects.—Medium to large hard-shelled insects such as moths, beetles, flies, bees, and wasps, should be pinned vertically through the body, fig. 11a. It is essential that the pin pass through a fairly solid part of the body, and, to insure this, the following standard procedures should be adopted:

1. Bees, wasps, flies.—Pin through thorax between bases of front wings slightly to right of middle line, fig. 11b.

2. Stink bugs.—Pin just to right of middle line of the scutellum or large triangle between the bases of the front wings, fig. 11c.

3. Grasshoppers.—Pin through back part of prothorax (the saddle behind the head) just to right of middle line, fig. 11d.

4. Beetles.—Pin near front margin of right wing cover near middle line, fig. 11e.

5. Moths, butterflies, dragonflies, damselflies.—Pin through the center of the thorax between the bases of the front wings, fig. 12.

The insect should be pushed about three-quarters of the distance up the pin, but not so close to the top that no room is left for easy handling of the pin with the fingers. It is well to have all insects the same distance from the top of the pin. To insure a uniform distance, the collector should use a pinning block. This is a small piece of wood or metal usually in the form shown in fig. 13, into the top of which are drilled holes slightly larger than the pin diameters. Such a block may be fashioned of wood with holes made by small nails and covered with a cardboard rectangle through which have been stabbed holes the exact size of those in the wood. The depths of the holes in the block should be three-eighths inch, three-quarters inch, and 1⅛ inches, respectively. To use the block, pin the insect and insert the pin into whichever hole allows the specimen to be pushed up the pin and still leave room, allowing for the thickness of the insect’s body, for handling at the top.

Tiny Insects.—Very small insects should be mounted on card points or on minuten pins. Regular pins would break too many of the insects’ body parts.

Card points are small triangles of cardboard or plastic pinned through one of the sides and crimped over at the opposite apex; a spot of strong glue is put on the angled tip, and the right side of the insect is pressed against the glued surface, fig. 14. The slant of the crimp depends on the angle of the insect’s side; the desired product is the insect mounted with its top surface horizontal and its head forward; legs, wings, and antennae should be in view and as little of the body as possible hidden by the glue or card point. Very little glue should be used; a small amount holds well and gives a better specimen for study than a large amount. The points may be cut uniformly with a hand punch, and they should be about three-eighths inch long. Good material for making these points is 2-ply Bristol board.

Minuten pins are short, extremely delicate steel pins, fig. 14d. One of these is thrust through the body of the insect and into a small piece of cork, pith, or similar substance, which is in turn pinned in the regular way a card point is. This method is especially desirable for tiny moths.

Insects Hard to Pin.—Wasps, lacewings, damselflies, and similar insects have an abdomen that sags readily when the specimen is killed and pinned. This unwanted drooping can be prevented in three simple ways. (1) Stick the pinned insect on a vertical surface of a block so that the body by its own weight dries in normal position. (2) Pin the insect on a horizontal surface and run a stiff paper on the pin beneath the body in such a way as to support it in a natural position until the insect dries. (3) Brace the abdomen by crossing two pins beneath it and thrusting them into the block, allowing the specimen to dry in the angle of the cross.

Crane flies are unwieldy and so are best pinned on a double card point mount, fig. 15. The legs should be directed away from the pin to avoid breakage in handling.

Spreading Board for Moths and Butterflies

Moths and butterflies should have their wings spread before being put into the collection. To do this well, it is necessary to have spreading boards that are accurately made but that are not necessarily complicated or expensive.

Construction.—A convenient board for medium-sized insects can be made at home of the following materials:

1.—A hardwood base, 4 × 12 × ¼ inch.

2.—Two hardwood end pieces, 4 × ¾ × ½ inch.

3.—Two softwood top pieces, 1⅞ × 12 × ½ inch, with the top surface planed at an angle, so that the thickness at one edge is ½ inch and at the other ⅜ inch.

4.—Two flat cork pieces 1 × 11 × ³/₁₆ inch.

Nail the top pieces to the ends so that the slanting surfaces of the tops are uppermost and the narrower edges parallel and one-quarter inch apart, fig. 16. Glue one strip of cork beneath the top pieces, covering the opening between and fitting snugly at each end. Glue the other cork piece flat to the upper side of the base, lengthwise along the middle, and extending to within one-half inch of each end. Finally, nail the base across the bottoms of the end pieces, so that the two corks face each other.

Use.—Before spreading the specimen, relax it as described under “Relaxing Boxes and Jars.” Then pin it, keeping in mind fig. 12 and the directions given under “Preservation by Pinning.” Thrust the pin, with the insect on it, through the upper cork of the board and into the cork on the base. Insert the insect body in the groove so that the wing bases are level with the near edge of each top piece. Hold the wings at the top level by two narrow strips of paper and pull them forward until the hind margin of the front wing is at right angles to the body axis, and the front margin of the hind wing is just under the front wing, fig. 16. Pin the wings temporarily in this position by inserting a pin, size 0 or 00, near the front margin at the base of each wing. When the wings on both sides of the insect are thus adjusted, lay strong pieces of paper over them and pin them down securely with large pins inserted close to the wings but not through them. Here you may use large common pins, but still better are the large-headed dressmaker’s pins about 1¼ inches long. Finally, remove the original adjusting pins and put the specimen in a dry, pest-proof container for 2 or 3 weeks. It will then have set sufficiently to be removed from the board.

For good results, spreading boards with grooves of various widths are necessary; a specimen should be spread on a board with a groove that fits the body. The width of the top pieces should vary to accommodate different wingspreads. The slope of the top pieces should be about as described.

Preservation in Fluid

Caterpillars and other immature stages of insects should be preserved in 80 per cent grain alcohol. Caterpillars, grubs, and maggots should first be heated 5 to 10 minutes in water just at the boiling point. This treatment sterilizes the specimens and prevents their discoloration by bacteria in the digestive system.

Many soft-bodied adult insects, including bristletails, springtails, stoneflies, and caddisflies, also should be preserved in fluid. If pinned, they shrivel to such an extent that few identifying characters can be seen. The preserving fluid in the vials in which insects have been placed should be changed at the end of the first day or two.

Some hard-shelled insects may be preserved in fluid. Ants and beetles may be thus treated temporarily and later pinned and dried.

HOW TO LABEL THE SPECIMENS

To be useful to the entomologist and others interested in the scientific relations of insects, as well as to furnish the collector with a complete record of his hours in the field and make more valuable the work he has already accomplished, the specimens should be labeled. The important information to be put on the label of each specimen is the locality and date of capture, but greater scientific value will be attached to the specimen by adding the name of the collector and the host on which the insect was found, or the particular habitat in which the insect was caught.

Labels should be made of a good grade of white paper stiff enough to stay flat when pierced and pushed up the pins. A very satisfactory high quality paper is available under the name “substance 36 ledger.” The labels may be printed by hand with a crow-quill pen and black India ink, or they may be purchased completely or partially printed from a biological supply house. They should be as small as possible and of nearly uniform size. They should be pushed up the pins, fig. 14, not too near the specimens, and they should project from the pins in the same direction as the specimens. To keep the labels small, yet to include all desirable information, it is often well to record the locality, collection date, and collector on one label, and the host plant or other pertinent information on a second label, fig. 14.

When the specimen is identified, its name should be recorded on still another label, which should be kept low on the pin. Sample identification labels are illustrated by the bottom labels in fig. 14.

HOUSING THE COLLECTION PERMANENTLY

After the specimens have been pinned and labeled, they should be housed in boxes or cases having a soft bottom or inner layer that will allow easy pinning. Such housing not only insures the safety of the collection but makes for easily handled units once the specimens have been named.

Insect Boxes

Several satisfactory types of boxes for housing insect specimens may be bought from commercial supply companies. These are usually much better than boxes of home construction, being more nearly dustproof and pestproof. Homemade boxes, however, are quite practical for the beginning collector, due to their ease of construction and extremely low cost. Cigar boxes 2 inches deep or more make ideal insect boxes if a layer of cork or balsa wood or two layers of soft, corrugated cardboard are glued in the bottom. Other wooden or cardboard boxes may be provided with such a bottom pinning surface and used for storing specimens. Boxes of this type, however, afford the specimens no protection against pests, and great care must be exercised in keeping the boxes fumigated.

Manufactured boxes, cabinets, and cases may be selected from catalogs that various scientific supply firms send free upon application.

Precaution Against Pests

Certain insects, such as flour beetles and carpet beetles, feed upon dried insects, and unless precautions are taken these may entirely destroy a collection. To guard against them, various chemical repellents may be placed in the boxes containing specimens. Naphthalene, of which ordinary mothballs are composed, is one of the best repellents. A few mothballs may be put in a cloth bag pinned securely in one corner of the box, or the heads of common pins may be inserted into naphthalene mothballs, and the points stuck in the corners of the box, fig. 17.

Naphthalene is chiefly repellent in action; its odor keeps out pests, but, if they are already in the specimen boxes, naphthalene will usually not kill these pests, and some other substance must be used.

Paradichlorobenzene, called PDB, is a good fumigant to use on pests in the collection. It should be used in a nearly airtight container, such as a tight trunk, bin, or case, at the rate of 1 pound of PDB to 25 cubic feet of space. The boxes of specimens, with lids open or removed, should be placed in the container, the fumigant scattered or spread on a piece of cloth or paper above them, and the container sealed for about a week.

THE INSECT WORLD

When the insects have been collected, mounted or preserved, and labeled, the next step is to identify or name them. This is no easy task, because there are so many different kinds of insects. In the whole world there are well over 1 million different kinds and in Illinois alone probably 20,000 different kinds.

The identification of insects is simplified somewhat by the fact that many species are closely related and can be classified into a number of major groups. Insects as a whole constitute what is called a class of animals, the Insecta. The crabs, shrimps, and their allies constitute a class called the Crustacea; the snakes, turtles, lizards, and their allies constitute another class called the Reptilia; and so on. The entire insect class is divided into orders, such as the Coleoptera, or beetles, the Diptera, or flies, and the Siphonaptera, or fleas. Each of these orders may contain several dozen to 25,000 different kinds of insects in North America alone. These orders are divided into families, each of which may contain one species to many thousands of species. The family names always end in -idae, as in Pentatomidae, the name for the stink bugs. The families are divided into genera (the plural for genus), and the various species (the word is the same for both singular and plural) or kinds are placed in the genera.

The house fly bears the name Musca domestica Linnaeus; this means that the species name is domestica, that the name was first applied to the species by Carolus Linnaeus (known as the describer of the insect or the author of the name), and that the species domestica is in the genus Musca. The genus Musca belongs to the family Muscidae, which, in turn, belongs to the order Diptera of the class Insecta.

Scientists may decide that a certain species belongs in another genus. When the species is transferred from the genus in which it was originally described to another genus, the name of the author is placed in parentheses. For example, the chinch bug was originally described by Thomas Say in the genus Lygaeus and had the name Lygaeus leucopterus Say. Later the species leucopterus was transferred to the genus Blissus, and Say’s name was placed in parentheses, thus: Blissus leucopterus (Say).

In the process of growth, insects go through a series of interesting stages. When the immature insect reaches a certain size, its outside skin covering or cuticle will not stretch further and the insect then acquires a larger cuticle by a process called molting.

Molting consists of a definite sequence of steps: (1) A goodly portion of the inside layer of the cuticle is dissolved by special glands situated among cells immediately below the cuticle; (2) the cells under the cuticle then exude material which forms a new cuticle beneath what is left of the old cuticle; (3) when the new cuticle is completely formed, the insect breaks a slit in the old cuticle, crawls out of it, and leaves it behind in the form of a cast skin; (4) the insect goes through many contortions, during which the soft parts of the new cuticle are stretched to a larger size than the corresponding parts of the old one; (5) the cuticle becomes set and unstretchable almost immediately, and the insect resumes its normal activities. During the molting process, the hard plates of each new cuticle are formed a size larger than the corresponding parts of the old cuticle, and the soft parts are stretched a size larger than the old. When the insect resumes its normal activities immediately after a molt, the soft parts of the cuticle fall into a large number of pleats or folds between the hard parts. As the insect grows larger following a molt, the body can lengthen by the unfolding of these pleated areas.

The stages of the insect between molts are called instars. Among the different orders of insects the number of instars in the life history may vary, and various instars may have different forms. These characteristics of molting and instars are therefore important items in the classification of insects.

Larvae and pupae evolved

MOTHS, BUTTERFLIES

FLIES

ANTS, BEES, WASPS

BEETLES

CADDISFLIES

LACEWINGS

FLEAS

SCORPIONFLIES

ALDERFLIES

Wing folding evolved

BUGS

GRASSHOPPERS

THRIPS

EARWIGS

LICE

TERMITES

BARKLICE

MANTIDS

COCKROACHES

ZORAPTERANS

STONEFLIES

Wings evolved

DRAGONFLIES

MAYFLIES

Primitive wingless insects

SPRINGTAILS

PROTURANS

SILVERFISH

CAMPODEIDS

BRISTLETAILS

The insect orders are arranged in a classification based on the sequence in which the orders are believed to have evolved, fig. 18. Measured by geological time, insects are among the oldest of land animals, having first evolved from an earlier, centipede-like ancestor about 400 million years ago. The first insects had no wings and differed from the many-legged centipede-like creatures of that time chiefly in having only three pairs of functional legs. The legs were situated on the three segments immediately behind the head; the three distinctive segments are together called the thorax. The part of the body behind the thorax is called the abdomen. In contrast to this arrangement, a centipede has a pair of legs on each of its many segments for the whole length of the body. The slow evolutionary change from such a many-legged ancestral form to a typical insect undoubtedly occurred by a gradual enlargement and strengthening of the front three pairs of legs and a reduction of the legs posterior to these. Evidence supporting this idea is found in insect embryos, which normally have rudimentary leg structures on the abdominal segments, and in some of the extremely primitive insects, which have rudimentary legs called styli on some segments of the abdomen. The result of this evolutionary development is a body having the front part, the thorax, specialized for locomotion and the back part, the abdomen, serving chiefly as a container for the vital organs, such as those of the digestive and reproductive systems.