When we consider the reputedly poisonous larvæ of moths and butterflies, one of the first things to impress us is that we cannot judge by mere appearance. Various species of Sphingid, or hawk-moth larvæ, bear at the end of the body a chitinous horn, which is often referred to as a "sting" and regarded as capable of inflicting dangerous wounds. It would seem unnecessary to refer to this absurd belief if it were not that each summer the newspapers contain supposed accounts of injury from the "tomato worm" (fig. 29) and others of this group. The grotesque, spiny larva (fig. 30) of one of our largest moths, Citheronia regalis is much feared though perfectly harmless, and similar instances could be multiplied.

But if the larvæ are often misjudged on account of their ferocious appearance, the reverse may be true. A group of most innocent looking and attractive caterpillars is that of the flannel-moth larvæ, of which Lagoa crispata may be taken as an example. Its larva (fig. 31) has a very short and thick body, which is fleshy and completely covered and hidden by long silken hairs of a tawny or brown color, giving a convex form to the upper side. Interspersed among these long hairs are numerous short spines connected with underlying hypodermal poison glands. These hairs are capable of producing a marked nettling effect when they come in contact with the skin. This species is found in our Atlantic and Southern States. Satisfactory studies of its poisonous hairs and their glands have not yet been made.

Sibine stimulea (Empretia stimulea), or the saddle-back caterpillar (fig. 32), is another which possesses nettling hairs. This species belongs to the group of Eucleidæ, or slug caterpillars. It can be readily recognized by its flattened form, lateral, bristling spines and by the large green patch on the back resembling a saddle-cloth, while the saddle is represented by an oval, purplish-brown spot. The small spines are venomous and affect some persons very painfully. The larva feeds on the leaves of a large variety of forest trees and also on cherry, plum, and even corn leaves. It is to be found throughout the Eastern and Southern United States.

Automeris io is the best known of the nettling caterpillars. It is the larva of the Io moth, one of the Saturniidæ. The mature caterpillar, (fig. 33), which reaches a length of two and one-half inches, is of a beautiful pale green with sublateral stripes of cream and red color and a few black spines among the green ones. The green radiating spines give the body a mossy appearance. They are tipped with a slender chitinous hair whose tip is readily broken off in the skin and whose poisonous content causes great irritation. Some individuals are very susceptible to the poison, while others are able to handle the larvæ freely without any discomfort. The larvæ feed on a wide range of food plants. They are most commonly encountered on corn and on willow, because of the opportunities for coming in contact with them.

The larvæ of the brown-tail moth (Euproctis chrysorrhœa) (fig. 35 and 36), where they occur in this country, are, on account of their great numbers, the most serious of all poisonous caterpillars. It is not necessary here, to go into details regarding the introduction of this species from Europe into the New England States. This is all available in the literature from the United States Bureau of Entomology and from that of the various states which are fighting the species. Suffice to say, there is every prospect that the pest will continue to spread throughout the Eastern United States and Canada and that wherever it goes it will prove a direct pest to man as well as to his plants.

Very soon after the introduction of the species there occurred in the region where it had gained a foothold, a mysterious dermatitis of man. The breaking out which usually occurred on the neck or other exposed part of the body was always accompanied by an intense itching. It was soon found that this dermatitis was caused by certain short, barbed hairs of the brown-tail caterpillars and that not only the caterpillars but their cocoons and even the adult female moths might harbor these nettling hairs and thus give rise to the irritation. In many cases the hairs were wafted to clothing on the line and when this was worn it might cause the same trouble. Still worse, it was found that very serious internal injury was often caused by breathing or swallowing the poisonous hairs.

The earlier studies seemed to indicate that the irritation was purely mechanical in origin, the result of the minute barbed hairs working into the skin in large numbers. Subsequently, however, Dr. Tyzzer (1907) demonstrated beyond question that the trouble was due to a poison contained in the hairs. In the first place, it is only the peculiar short barbed hairs which will produce the dermatitis when rubbed on the skin, although most of the other hairs are sharply barbed. Moreover, it was found that in various ways the nettling properties could be destroyed without modifying the structure of the hairs. This was accomplished by baking for one hour at 110° C, by warming to 60° C in distilled water, or by soaking in one per cent. or in one-tenth per cent. of potassium hydrate or sodium hydrate. The most significant part of his work was the demonstration of the fact that if the nettling hairs are mingled with blood, they immediately produce a change in the red corpuscles. These at once become coarsely crenated, and the roleaux are broken up in the vicinity of the hair (fig. 37b). The corpuscles decrease in size, the coarse crenations are transformed into slender spines which rapidly disappear, leaving the corpuscles in the form of spheres, the light refraction of which contrasts them sharply with the normal corpuscles. The reaction always begins at the basal sharp point of the hair. It could not be produced by purely mechanical means, such as the mingling of minute particles of glass wool, the barbed hairs of a tussock moth, or the other coarser hairs of the brown-tail, with the blood.

The question of the source of the poison has been studied in our laboratory by Miss Cornelia Kephart. She first confirmed Dr. Tyzzer's general results and then studied carefully fixed specimens of the larvæ to determine the distribution of the hairs and their relation to the underlying tissues.

The poison hairs (fig. 37), are found on the subdorsal and lateral tubercles (fig. 38), in bunches of from three to twelve on the minute papillæ with which the tubercles are thickly covered. The underlying hypodermis is very greatly thickened, the cells being three or four times the length of the ordinary hypodermal cells and being closely crowded together. Instead of a pore canal through the cuticula for each individual hair, there is a single pore for each papillæ on a tubercle, all the hairs of the papilla being connected with the underlying cells through the same pore canal, (figs. 39 and 40).

The hypodermis of this region is of two distinct types of cells. First, there is a group of slender fusiform cells, one for each poison hair on the papilla, which are the trichogen, or hair-formative cells. They are crowded to one side and towards the basement membrane by a series of much larger, and more prominent cells (fig. 40), of which there is a single one for each papilla. These larger cells have a granular protoplasm with large nuclei and are obviously actively secreting. They are so characteristic in appearance as to leave no question but that they are the true poison glands.

Poisonous larvæ of many other species have been reported from Europe and especially from the tropics but the above-mentioned species are the more important of those occurring in the United States and will serve as types. It should be noted in this connection that through some curious misunderstanding Gœldi (1913) has featured the larva of Orgyia leucostigma, the white-marked tussock moth, as the most important of the poisonous caterpillars of this country. Though there are occasional reports of irritation from its hairs such cases are rare and there is no evidence that there is any poison present. Indeed, subcutaneous implantation of the hairs leads to no poisoning, but merely to temporary irritation.

Occasionally, the hairs of certain species of caterpillars find lodgement in the conjunctiva, cornea, or iris of the eye of man and give rise to the condition known as opthalmia nodosa. The essential feature of this trouble is a nodular conjunctivitis which simulates tuberculosis of the conjunctiva and hence has been called pseudo-tubercular. It may be distinguished microscopically by the presence of the hairs.

Numerous cases of opthalmia nodosa are on record. Of those from this country, one of the most interesting is reported by de Schweinitz and Shumway (1904). It is that of a child of fifteen years whose eye had become inflamed owing to the presence of some foreign body. Downward and inward on the bulbar conjunctiva were a number of flattened, grayish-yellow nodules, between which was a marked congestion of the conjunctival and episcleral vessels (fig. 41a). Twenty-seven nodules could be differentiated, those directly in the center of the collection being somewhat confluent and assuming a crescentic and circular appearance. The nodules were excised and, on sectioning, were found to be composed of a layer of spindle cells and round cells, outside of which the tissue was condensed into a capsule. The interior consisted of epithelioid cells, between which was a considerable intercellular substance. Directly in the center of a certain number of nodules was found the section of a hair (fig. 41b). The evidence indicated that the injury had resulted from playing with caterpillars of one of the Arctiid moths, Spilosoma virginica. Other reported cases have been caused by the hairs of larvæ of Lasiocampa rubi, L. pini, Porthetria dispar, Psilura monacha and Cnethocampa processionea.

Relief from Poisoning by Nettling Larvæ—The irritation from nettling larvæ is often severe and, especially in regions where the brown-tail abounds, inquiries as to treatment arise. In general, it may be said that cooling lotions afford relief, and that scratching, with the possibilities of secondary infection, should be avoided, in so far as possible.

Among the remedies usually at hand, weak solutions of ammonia, or a paste of ordinary baking soda are helpful. Castellani and Chalmers recommend cleaning away the hairs by bathing the region with an alkaline lotion, such as two per cent solution of bicarbonate of soda, and then applying an ointment of ichthyol (10%).

In the brown-tail district, there are many proprietary remedies of which the best ones are essentially the following, as recommended by Kirkland (1907):

Shake thoroughly and rub well into the affected parts.

In some cases, and especially where there is danger of secondary infection, the use of a weak solution of creoline (one teaspoonful to a quart of water), is to be advised.

Vescicating Insects and those Possessing Other Poisons in their Blood Plasma

We have seen that certain forms, for example, the poisonous spiders, not only secrete a toxine in their poison glands, but that such a substance may be extracted from other parts of their body, or even their eggs. There are many insects which likewise possess a poisonous blood plasma. Such forms have been well designated by Taschenberg as cryptotoxic (κρυπτος = hidden). We shall consider a few representative forms.

The Blister Beetles—Foremost among the cryptotoxic insects are the Meloidæ or "blister beetles," to which the well-known "Spanish fly" (fig. 42a), formerly very generally used in medical practice, belongs. The vescicating property is due to the presence in the blood plasma of a peculiar, volatile, crystalline substance known as cantharidin, which is especially abundant in the reproductive organs of the beetle. According to Kobert, the amount of this varies in different species from .4 or .5% to 2.57% of the dry weight of the beetle.

While blister beetles have been especially used for external application, they are also at times used internally as a stimulant and a diuretic. The powder or extract was formerly much in vogue as an aphrodisiac, and formed the essential constituent of various philters, or "love powders". It is now known that its effects on the reproductive organs appear primarily after the kidneys have been affected to such an extent as to endanger life, and that many cases of fatal poison have been due to its ignorant use.

There are many cases on record of poisoning and death due to internal use, and in some instances from merely external application. There are not rarely cases of poisoning of cattle from feeding on herbage bearing a large number of the beetles and authentic cases are known of human beings who have been poisoned by eating the flesh of such cattle. Kobert states that the beetles are not poisonous to birds but that the flesh of birds which have fed on them is poisonous to man, and that if the flesh of chickens or frogs which have fed on the cantharidin be fed to cats it causes in them the same symptoms as does the cantharidin.

Treatment of cases of cantharidin poison is a matter for a skilled physician. Until he can be obtained, emetics should be administered and these should be followed by white of egg in water. Oils should be avoided, as they hasten the absorption of the poison.

Other Cryptotoxic Insects—Though the blister beetles are the best known of the insects with poisonous blood plasma, various others have been reported and we shall refer to a few of the best authenticated.

In China, a bug, Heuchis sanguinea, belonging to the family Cicadidæ, is used like the Meloidæ, to produce blistering, and often causes poisoning. It has been assumed that its vescicating properties are due to cantharidin, but the presence of this substance has not been demonstrated.

Certain Aphididæ contain a strongly irritating substance which produces, not merely on mucous membranes but on outer skin, a characteristic inflammation.

It has been frequently reported that the larvæ of the European cabbage butterfly, Pieris brassicæ, accidentally eaten by cows, horses, ducks, and other domestic animals, cause severe colic, attempts to vomit, paralysis of the hind legs, salivation, and stomatitis. On postmortem there are to be found hæmorrhagic gastro-enteritis, splenitis, and nephritis. Kobert has recently investigated the subject and has found a poisonous substance in the blood of not only the larvæ but also the pupæ.

CHAPTER III

Facultative parasites are forms which are not normally parasitic, but which, when accidentally ingested, or otherwise brought into the body, are able to exist for a greater or less period of time in their unusual environment. These are generally called in the medical literature "pseudoparasites" but the term is an unfortunate one.

We shall now take up the different groups of arthropods, discussing the more important of the parasitic forms attacking man. The systematic relationship of these forms, and key for determining important species will be found in Chapter XII.

Acarina or Mites

Of the many parasitic forms, there are several species which are serious parasites of man and we shall consider the more important of these. Infestation by mites is technically known as acariasis.

The Trombidiidæ, or Harvest Mites

In many parts of this country it is impossible for a visitor to go into the fields and, particularly, into berry patches and among tall weeds and grass in the summer or early fall without being affected by an intolerable itching, which is followed, later, by a breaking out of wheals, or papules, surrounded by a bright red or violaceous aureola, (fig. 43). It is often regarded as a urticaria or eczema, produced by change of climate, an error in diet, or some condition of general health.

Sooner or later, the victim finds that it is due to none of these, but to the attacks of an almost microscopic red mite, usually called "jigger" or "chigger" in this country. As the term "chigger" is applied to one of the true fleas, Dermatophilus penetrans, of the tropics, these forms are more correctly known as "harvest mites." Natives of an infested region may be so immune or accustomed to its attacks as to be unaware of its presence, though such immunity is by no means possessed by all who have been long exposed to the annoyance.

The harvest mites, or chiggers, attacking man are larval forms, possessing three pairs of legs (fig. 44). Their systematic position was at first unknown and they were classed under a special genus Leptus, a name which is very commonly still retained in the medical literature. It is now known that they are the larval forms of various species of the genus Trombidium, a group of predaceous forms, the adults of which feed primarily on insects and their eggs. In this country the species best known are those to be found late in summer, as larvæ at the base of the wings of houseflies or grasshoppers.

There is much uncertainty as to the species of the larvæ attacking man but it is clear that several are implicated. Bruyant has shown that in France the larvæ Trombidium inapinatum and Trombidium holosericeum are those most frequently found. The habit of attacking man is abnormal and the larvæ die after entering the skin. Normally they are parasitic on various insects.

Most recent writers agree that, on man, they do not bore into the skin, as is generally supposed, but enter a hair follicle or sebaceous gland and from the bottom of this, pierce the cutis with their elongate hypopharynx. According to Braun, there arises about the inserted hypopharynx a fibrous secretion—the so-called "beak" which is, in reality, a product of the host. Dr. J. C. Bradley, however, has made careful observations on their method of attack, and he assures us that the mite ordinarily remains for a long time feeding on the surface of the skin, where it produces the erythema above described. During this time it is not buried in the skin but is able to retreat rapidly into it through a hair follicle or sweat gland. The irritation from the mites ceases after a few days, but not infrequently the intolerable itching leads to so much scratching that secondary infection follows.

Relief from the irritation may be afforded by taking a warm salt bath as soon as possible after exposure or by killing the mites by application of benzine, sulphur ointment or carbolized vaseline. When they are few in number, they can be picked out with a sterile needle.

Much may be done in the way of warding off their attacks by wearing gaiters or close-woven stockings extending from ankle to the knee. Still more efficacious is the sprinkling of flowers of sulphur in the stockings and the underclothes from a little above the knee, down. The writers have known this to make it possible for persons who were especially susceptible to work with perfect comfort in badly infested regions. Powdered naphthalene is successfully used in the same way and as Chittenden (1906) points out, is a safeguard against various forms of man-infesting tropical insect pests.

The question of the destruction of the mites in the field is sometimes an important one, and under some conditions, is feasible. Chittenden states that much can be accomplished by keeping the grass, weeds, and useless herbage mowed closely, so as to expose the mites to the sun. He believes that in some cases good may be done by dusting the grass and other plants, after cutting, with flowers of sulphur or by spraying with dilute kerosene emulsion in which sulphur has been mixed. More recently (1914) he calls attention to the value of cattle, and more especially sheep, in destroying the pests by tramping on them and by keeping the grass and herbage closely cropped.

Ixodoidea or Ticks

The ticks are the largest of the Acarina. They are characterized by the fact that the hypostome, or "tongue" (fig. 45) is large and file-like, roughened by sharp teeth. They possess a breathing pore on each side of the body, above the third or fourth coxæ (fig. 45b).

There are two distinct families of the Ixodoidea, differing greatly in structure, life-history and habits. These are the Argasidæ and the Ixodidæ. We shall follow Nuttall (1908) in characterizing these two families and in pointing out their biological differences, and shall discuss briefly the more important species which attack man. The consideration of the ticks as carriers of disease will be reserved for a later chapter.

Argasidæ

In the ticks belonging to the family Argasidæ, there is comparatively little sexual dimorphism, while this is very marked in the Ixodidæ. The capitulum, or so-called "head" is ventral, instead of terminal; the palpi are leg-like, with the segments subequal; the scutum, or dorsal shield, is absent; eyes, when present, are lateral, on supracoxal folds. The spiracles are very small; coxæ unarmed; tarsi without ventral spurs, and the pulvilli are absent or rudimentary.

In habits and life history the Argasidæ present striking characteristics. In the first place, they are long-lived, a factor which counts for much in the maintenance of the species. They are intermittent feeders, being comparable with the bed-bug in this respect. There are two or more nymphal stages, and they may molt after attaining maturity. The female lays comparatively few eggs in several small batches.

Nuttall (1911) concludes that "The Argasidæ represent the relatively primitive type of ticks because they are less constantly parasitic than are the Ixodidæ. Their nymphs and adults are rapid feeders and chiefly infest the habitat of their hosts. * * * Owing to the Argasidæ infesting the habitats of their hosts, their resistance to prolonged starvation and their rapid feeding habits, they do not need to bring forth a large progeny, because there is less loss of life in the various stages, as compared with the Ixodidæ, prior to their attaining maturity."

Of the Argasidæ, we have in the United States, several species which have been reported as attacking man.

Argas persicus, the famous "Miana bug" (fig. 46), is a very widely distributed species, being reported from Europe, Asia, Africa, and Australia. It is everywhere preeminently a parasite of fowls. According to Nuttall it is specifically identical with Argas americanus Packard or Argas miniatus Koch, which is commonly found on fowls in the United States, in the South and Southwest. Its habits are comparable to those of the bed-bug. It feeds intermittently, primarily at night, and instead of remaining on its host, it then retreats to cracks and crevices. Hunter and Hooker (1908) record that they have found the larva to remain attached for five or eight days before dropping. Unlike the Ixodidæ, the adults oviposit frequently.

The most remarkable feature of the biology of this species is the great longevity, especially of the adult. Hunter and Hooker report keeping larvæ confined in summer in pill boxes immediately after hatching for about two months while under similar conditions those of the Ixodid, Boophilus annulatus lived for but two or three days. Many writers have recorded keeping adults for long periods without food. We have kept specimens in a tin box for over a year and a half and at the end of that time a number were still alive. Laboulliene kept unfed adults for over three years. In view of the effectiveness of sulphur in warding off the attacks of Trombidiidæ, it is astonishing to find that Lounsbury has kept adults of Argas persicus for three months in a box nearly filled with flowers of sulphur, with no apparent effect on them.

We have already called attention to the occasional serious effects of the bites of this species. While such reports have been frequently discredited there can be no doubt that they have foundation in fact. The readiness with which this tick attacks man, and the extent to which old huts may be infested makes it especially troublesome.

Otiobius (Ornithodoros) megnini, the "spinose ear-tick" (figs. 47, 48), first described from Mexico, as occurring in the ears of horses, is a common species in our Southwestern States and is recorded by Banks as occurring as far north as Iowa.

The species is remarkable for the great difference between the spiny nymph stage and the adult. The life history has been worked out by Hooker (1908). Seed ticks, having gained entrance to the ear, attach deeply down in the folds, engorge, and in about five days, molt; as nymphs with their spinose body they appear entirely unlike the larvæ. As nymphs they continue feeding sometimes for months. Finally the nymph leaves the host, molts to form the unspined adult, and without further feeding is fertilized and commences oviposition.

The common name is due to the fact that in the young stage the ticks occur in the ear of their hosts, usually horses or cattle. Not uncommonly it has been reported as occurring in the ear of man and causing very severe pain. Stiles recommends that it be removed by pouring some bland oil into the ear.

Banks (1908) reports three species of Ornithodoros—O. turicata, coriaceus and talaje—as occurring in the United States. All of these attack man and are capable of inflicting very painful bites.

Ixodidæ

The ticks belonging to the family Ixodidæ (figs. 49 and 50) exhibit a marked sexual dimorphism. The capitulum is anterior, terminal, instead of ventral as in the Argasidæ; the palpi are relatively rigid (except in the subfamily Ixodinæ), with rudimentary fourth segment; scutum present; eyes, when present, dorsal, on side of scutum. The spiracles are generally large, situated well behind the fourth coxæ; coxæ generally with spurs; pulvilli always present.

In habits and life history the typical Ixodidæ differ greatly from the Argasidæ. They are relatively short-lived, though some recent work indicates that their longevity has been considerably under-estimated. Typically, they are permanent feeders, remaining on the host, or hosts, during the greater part of their life. They molt twice only, on leaving the larval and the nymphal stages. The adult female deposits a single, large batch of eggs. Contrasting the habits of the Ixodidæ to those of the Argasidæ, Nuttall (1911) emphasizes that the Ixodidæ are more highly specialized parasites. "The majority are parasitic on hosts having no fixed habitat and consequently all stages, as a rule, occur upon the host."