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Our Common Insects Part 11

Verrill it is readily visible to the naked eye and swarms on horses afflicted with the mange, which is a disease analogous to the itch in man. It has a soft, depressed body, spiny beneath at the base of the legs and on the thorax. One or both of the two posterior pairs of feet bear suckers, and all are more or less covered with long, slender hairs.

This insect may be destroyed by the same remedies as are used for lice and for the human itch. The best remedy is probably a solution of sulphuret of potassium.

[Illustration: 150. Itch Mite.]

[Illustration: 151. Nose Mite.]

The itch insect (Sarcoptes scabiei, Fig. 150) was first recognized by an Arabian author of the twelfth century, as the cause of the disease which results from its attacks. The body of the insect is rounded, with the two hind pair of feet rudimentary and bearing long hairs. It buries itself in the skin on the more protected parts of the body, and by its punctures maintains a constant irritation. Other species are known to infest the sheep and dog. Another singular mite is the Demodex folliculorum (Fig. 151), which was discovered by Dr. Simon, of Berlin, buried in the diseased follicles of the wings of the nose in man. It is a long, slender, worm-like form, with eight short legs, and in the larva state has six legs. This singular form is one of the lowest and most degraded of the order of Arachnids. A most singular mite was discovered by Newport on the body of a larva of a wild bee, and described by him under the name of Heteropus ventricosus. The body of the fully formed female is long and slender. After attaining this form, its small abdomen begins to enlarge until it assumes a globular form, and the mass of mites look like little beads. Mr. Newport was unable to discover the male, and thought that this mite was parthenogenous. It will be seen that the adult Demodex retains the elongated, worm-like appearance of the larva of the higher mites, such as Typhlodromus. This is an indication of its low rank, and hints of a relationship to the Tardigrades and the Pentastoma, the latter being a degraded mite, and the lowest of its order, living parasitically within the bodies of other animals.

[Illustration: Harvestman.]

FOOTNOTES:

[Footnote 8: The figure at the bottom on the left represents the adult, fully-gorged tick.]

CHAPTER XII.

BRISTLE-TAILS AND SPRING-TAILS.

The Thysanura, as the Poduras and their allies, the Lepismas, are called, have been generally neglected by entomologists, and but few naturalists have paid special attention to them.[9] Of all those microscopists who have examined Podura scales as test objects, we wonder how many really know what a Podura is?

In preparing the following account I have been under constant indebtedness to the admirable and exhaustive papers of Sir John Lubbock, in the London "Linnaean Transactions" (vols. 23, 26 and 27).

Entomologists will be glad to learn that he is shortly going to press with a volume on the Poduras, which, in distinction from the Lepismas, to which he restricts the term Thysanura, he calls Collembola, in allusion to the sucker-like tubercle situated on the under side of the body, which no other insects are known to possess.

The group of Bristle-tails, as we would dub the Lepismas in distinction from the Spring-tails, we will first consider. They are abundant in the Middle States under stones and leaves in forests, and northward are common in damp houses, while one beautiful species that we have never noticed elsewhere, is our "cricket on the hearth," abounding in the chinks and crannies of the range of our house, and also in closets, where it feeds on sugar, etc., and comes out like cockroaches, at night, shunning the light. Like the cockroaches, which it vaguely resembles in form, this species loves hot and dry localities, in distinction from the others which seek moisture as well as darkness. By some they are called "silver witches," and as they dart off, when disturbed, like a streak of light, their bodies being coated in a suit of shining mail, which the arrangement of the scales resembles, they have really a weird and ghostly look.

The most complicated genus, and the one which stands at the head of the family, is Machilis, one species of which lives in the Northern and Middle States, and another in Oregon. They affect damp places, living under leaves and stones. They all have rounded, highly arched bodies, and large compound eyes, the two being united together. The maxillary palpi are greatly developed, but the chief characteristics are the two-jointed stylets arranged in nine pairs along each side of the abdomen, reminding us of the abdominal legs of Myriopods. The body ends in three long bristles, as in Lepisma.

The Lepisma saccharina of Linnaeus, if, as is probable, that is the name of our common species, is not uncommon in old damp houses, where it has the habits of the cockroach, eating cloths, tapestry, silken trimmings of furniture, and doing occasional damage to libraries by devouring the paste, and eating holes in the leaves and covers of books.

In general form Lepisma may be compared to the larva of Perla, a net-veined Neuropterous insect, and also to the narrow-bodied species of cockroaches, minus the wings. The body is long and narrow, covered with rather coarse scales, and ends in three many jointed anal stylets, or bristles, which closely resemble the many jointed antennae, which are remarkably long and slender. The thermophilous species already alluded to may be described as perhaps the type of the genus, the L. saccharina being simpler in its structure. The body is narrow and flattened; the basal joints of the legs being broad, flat and almost triangular, like the same joints in the cockroaches. The legs consist of six joints, the tarsal joints being large and two in number, and bearing a pair of terminal curved claws. The three thoracic segments are of nearly equal size, and the eight abdominal segments are also of similar size. The tracheae are well developed, and may be readily seen in the legs. The end of the rather long and weak abdomen is propped up by two or three pairs of bristles, which are simple, not jointed, but moving freely at their insertion; thus they take the place of legs, and remind one of the abdominal legs of the Myriopods; and we shall see in certain other genera (Machilis and Campodea) of the Bristle-tails that there are actually two-jointed bristles arranged in pairs along the abdomen. They may probably be directly compared with the abdominal legs of Myriopods.

Further study, however, of the homologies of these peculiar appendages, and especially a knowledge of the embryological development of Lepisma and Machilis, is needed before this interesting point can be definitely settled. The three many jointed anal stylets may, however, be directly compared with the similar appendages of Perla and Ephemera. The mode of insertion of the antennae of this family is much like that of the Myriopods, the front of the head being flattened, and concealing the base of the antennae, as in the Centipedes and Pauropus. Indeed, the head of any Thysanurous insect seen from above, bears a general resemblance in some of its features to that of the Centipede and its allies. So in a less degree does the head of the larvae of certain Neuroptera and Coleoptera. The eyes are compound, the single facets forming a sort of heap. The clypeus and labrum, or upper lip, is, in all the Thysanura, carried far down on the under side of the head, the clypeus being almost obsolete in the Poduridae, this being one of the most essential characters of that family. Indeed, it is somewhat singular that these and other important characteristics of this group have been almost entirely passed over by authors, who have consequently separated these insects from other groups on what appear to the writer as comparatively slight and inconsiderable characters. The mouth-parts of the Lepismatidae (especially the thermophilous Lepisma, which we now describe) are most readily compared with those of the larva of Perla. The rather large, stout mandibles are concealed at their tips, under the upper lip, which moves freely up and down when the creature opens its mouth. The mandible is about one-third as broad as long, armed with three sharp teeth on the outer edge, and with a broad cutting edge within, and still further inwards a lot of straggling spinules. In all these particulars, the mandible of Lepisma is comparable with that of certain Coleoptera and Neuroptera. So also are the maxillae and labium, though we are not aware that any one has indicated how close the homology is. The accompanying figure (152) of the maxilla of a beetle may serve as an example of the maxilla of the Coleoptera, Orthoptera and Neuroptera. In these insects it consists almost invariably of three lobes, the outer being the palpus, the middle lobe the galea, and the innermost the lacinia; the latter undergoing the greatest modifications, forming a comb composed of spines and hairs varying greatly in relative size and length. How much the palpi vary in these groups of insects is well known. The galea sometimes forms a palpus-like appendage. Now these three lobes may be easily distinguished in the maxilla of Lepisma. The palpus instead of being directed forward, as in the insects mentioned above (in the pupa of Ephemera the maxilla is much like that of Lepisma), is inserted nearer the base than usual and thrown off at right angles to the maxilla, so that it is stretched out like a leg, and in moving about the insect uses its maxillae partly as supports for its head. They are very long and large, and five or six-jointed. The galea, or middle division, forms a simple lobe, while the lacinia has two large chitinous teeth on the inner edge, and internally four or five hairs arising from a thin edge.

[Illustration: 152. Maxilla.]

The labium is much as in that of Perla, being broad and short, with a distinct median suture, indicating its former separation in embryonic life into a pair of appendages. The labial palpi are three-jointed, the joints being broad, and in life directed backwards instead of forwards as in the higher insects.

There are five American species of the genus Lepisma in the Museum of the Peabody Academy. Besides the common L. saccharina? there are four undescribed species; one found about out-houses and cellars, and the heat-loving form, perhaps an imported, species, found in a kitchen in Salem, and apparently allied to the L. thermophila Lucas, of houses in Brest, France; and lastly two allied forms, one from Key West, and another from Polvon, Western Nicaragua, collected by Mr. McNiel. The last three species are beautifully ornamented with finely spinulated hairs arranged in tufts on the head; while the sides of the body, and edges of the basal joints of the legs are fringed with them.

The interesting genus Nicoletia stands at the bottom of the group. It has the long, linear, scaleless body of Campodea, in the family below, but the head and its appendages are like Lepisma, the maxillary palpi being five-jointed, and the labial palpi four-jointed. The eyes are simple, arranged in a row of seven on each side of the head. The abdomen ends in three long and many jointed stylets, and there are the usual "false branchial feet" along each side of the abdomen. There are two European species which occur in greenhouses. No species have yet been found in America.

[Illustration: 153. Japyx solifugus.]

The next family of Thysanura is the Campodeae, comprising the two genera Campodea and Japyx. These insects are much smaller than the Lepismidae, and in some respects are intermediate between that family and the Poduridae (including the Smynthuridae).

In this family the body is long and slender, and the segments much alike in size. There is a pair of spiracles on each thoracic ring. The mandibles are long and slender, ending in three or four teeth, and with the other appendages of the mouth are concealed within the head, "only the tips of the palpi (and of the maxillae when these are opened) projecting a very little beyond the rounded entire margin of the epistoma," according to Haliday. The maxillae are comb-shaped, due to the four slender, minutely ciliated spines placed within the outer tooth.

The labium in Japyx is four-lobed and bears a small two-jointed palpus.

The legs are five-jointed, the tarsi consisting of a single joint, ending in two large claws. The abdomen consists of ten segments, and in Campodea along each side is a series of minute, two-jointed appendages such as have been described in Machilis. These are wanting in Japyx.

None of the species in this family have the body covered with scales.

They are white, with a yellowish tinge.

The more complicated genus of the two is Japyx (Fig. 153, Japyx solifugus, found under stones in Southern Europe; _a_, the mouth from beneath, with the maxillae open; _b_, maxilla; _d_, mandible; _c_, outline of front of head seen from beneath, with the labial palpi in position) which, as remarked by the late Mr. Haliday (who has published an elaborate essay on this genus in the Linnaean Transactions, vol. 24, 1864), resembles Forficula in the large forceps attached to its tail. An American species (J. Saussurii) lives in Mexico, and we look for its discovery in Texas.

[Illustration: 154. Campodea staphylinus.]

Campodea (C. staphylinus Westw., Fig. 154, enlarged; _a_, mandible; _b_, maxilla), otherwise closely related, has more rudimentary mouth-parts, and the abdomen ends in two many jointed appendages.

[Illustration: Fig. 155. Larva of Perla.]

Our common American species of Campodea (C. Americana) lives under stones in damp places. It is yellowish, about a sixth of an inch in length, is very agile in its movements, and would easily be mistaken for a very young Lithobius. A larger species and differing in having longer antennae, has been found by Mr. C. Cooke in Mammoth Cave, and has been described in the "American Naturalist" under the name of Campodea Cookei. Haliday has remarked that this family bears much resemblance to the Neuropterous larva of Perla (Fig. 155), as previously remarked by Gervais; and the many points of resemblance of this family and the Lepismidae to the larval forms of some Neuroptera that are active in the pupa state (the Pseudoneuroptera of Erichson and other authors) are very striking. Campodea resembles the earliest larval form of Chloeon, as figured by Sir John Lubbock, even to the single jointed tarsus; and why these two Thysanurous families should be removed from the Neuroptera we are unable, at present, to understand, as to our mind they scarcely diverge from the Neuropterous type more than the Mallophaga, or biting lice, from the type of Hemiptera.

Haliday, remarking on the opinion of Linnaeus and Schrank, who referred Campodea to the old genus Podura, says with much truth, "it may be perhaps no unfair inference to draw, that the insect in question is in some measure intermediate between both," _i. e._, Podura and Lepisma.

This is seen especially in the mouth-parts which are withdrawn into the head, and become very rudimentary, affording a gradual passage into the mouth-parts of the Poduridae, which we now describe.

The next group, the Podurelles of Nicolet, and Collembola of Lubbock, are considered by the latter, who has studied them with far more care than any one else, as "less closely allied" to the Lepismidae "than has hitherto been supposed." He says "the presence of tracheae, the structure of the mouth and the abdominal appendage; all indicate a wide distinction between the Lepismidae and the Poduridae. We must, indeed, in my opinion, separate them entirely from one another; and I would venture to propose for the group comprised in the old genus Podura, the term Collembola, as indicating the existence of a projection, or mammilla, enabling the creature to attach or glue itself to the body on which it stands." Then without expressing his views as to the position and affinities of the Lepismidae, he remarks "as the upshot of all this, then, while the Collembola are clearly more nearly allied to the Insecta than to the Crustacea or Arachnida, we cannot, I think, regard them as Orthoptera or Neuroptera, or even as true insects. That is to say, the Coleoptera, Orthoptera, Neuroptera, Lepidoptera, etc., are in my opinion, more nearly allied to one another than they are to the Poduridae or Smynthuridae. On the other hand, we certainly cannot regard the Collembola as a group equivalent in value to the Insecta. If, then, we attempt to map out the Articulata, we must, I think, regard the Crustacea and Insecta as continents, the Myriopoda and Collembola as islands--of less importance, but still detached. Or, if we represent the divisions of the Articulata like the branching of a tree, we must picture the Collembola as a separate branch, though a small one, and much more closely connected with the Insecta than with the Crustacea or the Arachnida." Lamarck regarded them as more nearly allied to the Crustacea than Insecta. Gervais, also, in the "Histoire Naturelle des Insectes: Apteres," indicates a considerable diversity existing between the Lepismidae and Poduridae, though they are placed next to each other.

Somewhat similar views have been expressed by so high an authority as Professor Dana, who, in the "American Journal of Science" (vol. 37, Jan., 1864), proposed a classification of insects based on the principle of cephalization, and divided the Hexapodous insects into three groups: the first (Ptero-prosthenics, or Ctenopters) comprising the Hymenoptera, Diptera, Aphaniptera (fleas), Lepidoptera, Homoptera, Trichoptera and Neuroptera; the second group (Ptero-metasthenics, or Elytropters) comprising the Coleoptera, Hemiptera and Orthoptera; while the Thysanura compose the third group. Lubbock has given us a convenient historical view of the opinions of different authors regarding the classification of these insects, which we find useful. Nicolet, the naturalist who, previous to Lubbock, has given us the most correct and complete account of the Thysanura, regarded them as an order, equivalent to the Coleoptera or Diptera, for example. In this he followed Latreille, who established the order in 1796. The Abbe Bourlet adopted the same view.

On the other hand Burmeister placed the Thysanura as a separate tribe between the Mallophaga (Bird Lice) and Orthoptera, and Gerstaecker placed them among the Orthoptera. Fabricius and Blainville put them with the Neuroptera, and the writer, in his "Guide to the Study of Insects,"

and previously in 1863, ignorant of the views of the two last named authors, considered the Thysanura as degraded Neuroptera, and noticed their resemblance to the larvae of Perla, Ephemera, and other Neuroptera, such as Rhaphidia and Panorpa, regarding them as standing "in the same relation to the rest of the Neuroptera [in the Linnaean sense], as the flea does to the rest of the Diptera, or the lice and Thrips to the higher Hemiptera."

After having studied the Thysanura enough to recognize the great difficulty of deciding as to their affinities and rank, the writer does not feel prepared to go so far as Dana and Lubbock, for reasons that will be suggested in the following brief account of the more general points in their structure, reserving for another occasion a final expression of his views as to their classification.

The Poduridae, so well known by name, as affording the scales used by microscopists as test objects, are common under stones and wet chips, or in damp places, cellars, mushrooms and about manure heaps. They need moisture, and consequently shade. They abound most in spring and autumn, laying their eggs at both seasons, though most commonly in the spring.

During a mild December, they may be found in abundance under sticks and stones, even in situations so far north as Salem, Mass.

[Illustration: 156. Smynthurus.]

The body of the Poduras is rather short and thick, most so in Smynthurus (Fig. 156), and becoming long and slender in Tomocerus and Isotoma. The segments are inclined to be of unequal size, the prothoracic ring sometimes becoming almost obsolete, and some of the abdominal rings are much smaller than others; while in Lipura and Anura, the lowest forms of the group, the segments are all much alike in size.

The head is in form much like that of certain larvae of Neuroptera and of Forficula, an Orthopterous insect. The basal half of the head is marked off from the eye-bearing piece (epicranium) by a V-shaped suture[10]

(Fig. 157, head of Degeeria; compare also the head of the larva of Forficula, Fig. 158, in which the suture is the same), and the insertion of the antennae is removed far down the front, near the mouth, the clypeus being very short; this piece, so large and prominent in the higher insects, is not distinctly separated by suture from the surrounding parts of the head, thus affording one of the best distinctive characters of the Poduridae. The eyes are situated on top of the head just behind the antennae, and are simple, consisting of a group of from five to eight or ten united into a mass in Smynthurus, but separated in the Poduridae (see Fig. 176, _e_, eye of Anurida). The antennae are usually four-jointed, and vary in length in the different genera.

[Illustration: 157. Head of Degeeria.]

[Illustration: 158. Larva of Forficula.]

The mouth-parts are very difficult to make out, but by soaking the insect in potash for twenty-four hours, thus rendering the body transparent, they can be satisfactorily observed. They are constructed on the same general type as the mouth-parts of the Neuroptera, Orthoptera and Coleoptera, and except in being degraded, and with certain parts obsolete, they do not essentially differ.[11] On observing the living Podura, the mouth seems a simple ring, with a minute labrum and groups of hairs and spinules, which the observer, partly by guess-work, can identify as jaws and maxillae, and labium. But in studying the parts rendered transparent, we can identify the different appendages. Figure 159 shows the common Tomocerus plumbeus greatly enlarged (Fig. 160, seen from above), and as the mouth-parts of the whole group of Poduras are remarkably constant, a description of one genus will suffice for all. The labrum, or upper lip, is separated by a deep suture from the clypeus, and is trapezoidal in form. The mandibles and maxillae are long and slender, and buried in the head, with the tips capable of being extended out from the ring surrounding the mouth for a very short distance. The mandibles (_md_, Fig. 159) are like those of the Neuroptera, Orthoptera and Coleoptera in their general form, the tip ending in from three to six teeth (three on one mandible and six on the other), while below, is a rough, denticulated molar surface, where the food seized by the terminal teeth is triturated and prepared to be swallowed. Just behind the mandibles are the maxillae, which are trilobate at the end, as in the three orders of insects above named. The outer lobe, or palpus, is a minute membranous tubercle ending in a hair (Fig. 161, _mp_), while the middle lobe, or galea, is nearly obsolete, though I think I have seen it in Smynthurus, where it forms a lobe on the outside of the lacinia. The lacinia, or inner lobe (Fig. 161, _lc_; 162, the same enlarged), in Tomocerus consists of two bundles of spinules, one broad like a ruffle, and the other slender, pencil-like, ending in an inner row of spines, like the spinules on the lacinia of the Japyx and Campodea and, more remotely, the laciniae of the three sub-orders of insects above referred to. There is also a horny, prominent, three-toothed portion (Fig. 161, _g_). These homologies have never been made before, so far as the writer is aware, but they seem natural, and suggested by a careful examination and comparison with the above-mentioned mandibulate insects.

[Illustration: 159. 161. 160. 162.

Tomocerus plumbeus and mouth-parts, greatly enlarged.]

The spring consists of a pair of three-jointed appendages, with the basal joints soldered together early in embryonic life, while the other two joints are free, forming a fork. It is longest in Smynthurus and Degeeria, and shortest in Achorutes (Fig. 172, _b_), where it forms a simple, forked tubercle; and is obsolete in Lipura and Anura, its place being indicated by an oval scar. The third joint varies in form, being hairy, serrate and knife-like in form, as in Tomocerus (Fig. 159, _a_), or minute, with a supplementary tooth, as in Achorutes (Fig. 172, _c_). This spring is in part homologous with the ovipositor of the higher insects, which originally consists of three pairs of tubercles, each pair arising apparently from the seventh, eighth, and ninth (the latter the penultimate) segments of the abdomen in the Hymenoptera. The spring of the Podura seems to be the homologue of the third pair of these tubercles, and is inserted on the penultimate segment. This comparison I have been able to make from a study of the embryology of Isotoma.

[Illustration: 163. Catch holding spring of Achorutes.]

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