Another organ, and one which, so far as I am aware, has been overlooked by previous observers, I am disposed to consider as possibly an ovipositor. In the genus Achorutes, it may be found in the segment just before the spring-bearing segment, and situated on the median line of the body. It consists (Fig. 163) of two squarish valves, from between which projects a pair of minute tubercles, or blades, with four rounded teeth on the under side. This pair of infinitesimal saws reminds one of the blades of the saw-fly, and I am at a loss what their use can be unless to cut and pierce so as to scoop out a shallow place in which to deposit an egg. It is h.o.m.ologous in situation with the middle pair of blades which composes the ovipositor of higher insects, and if it should prove to be used by the creature in laying its eggs, we should then have, with the spring, an additional point of resemblance to the Neuroptera and higher insects, and instead of this spring being an important differential character, separating the Thysanura from other insects, it binds them still closer, though still differing greatly in representing only a part of the ovipositor of the higher insects. (This is a catch for holding the spring in place.)
But all the Poduras differ from other insects in possessing a remarkable organ situated on the basal segment of the abdomen. It is a small tubercle, with chitinous walls, forming two valves from between which is forced out a fleshy sucker, or, as in Smynthurus, a pair of long tubes, which are capable of being darted out on each side of the body, enabling the insect to attach itself to smooth surfaces, and rest in an inverted position.
The eggs are laid few in number, either singly or several together, on the under side of stones, chips or, as in the case of Isotoma Walkerii, under the bark of trees. They are round, transparent. The development of the embryo of Isotoma in general accords with that of the Phryganeidae and suggests on embryological grounds the near relationship of the Thysanura to the Neuroptera.
[Ill.u.s.tration: 164. 165. 166. 167.
Development of a Poduran.]
The earliest stage observed was at the time of the appearance of the primitive band (Fig. 164, _a_, _b_, folding of the primitive band; _c_, the dotted line crosses the primitive band, and terminates in a large yolk granule) which surrounds the egg as in the Caddis flies. Soon after, the primitive segments appear (Fig. 165; 1, antennae; 2, mandibles; 3, maxillae; the labium was not seen; 5-7, legs; _c_, yolk surrounded by the primitive band) and seem to originate just as in the Caddis flies. Figure 166 is a front view of the embryo shortly before it is hatched; figure 167, side view of the same, the figures as in Fig.
165; _sp_, spring; _l_, labrum. The labrum or upper lip, and the clypeus are large and as distinct as in the embryos of other insects, a fact to which we shall allude again. The large three-jointed spring is now well developed, and the inference is drawn that it represents a pair of true abdominal legs. The embryo when about to hatch throws off the egg-sh.e.l.l and amnion in a few seconds. The larva is perfectly white and is very active in its movements, running over the damp, inner surface of the bark. It is a little over a hundredth of an inch in length, and differs from the adult in being shorter and thicker, with the spring very short and stout. In fact the larva a.s.sumes the form of the lower genera of the family, such as Achorutes and Lipura, the adult more closely resembling Degeeria. The larva after its first moult retains its early clumsy form, and is still white. After a second moult it becomes purplish, and much more slender, as in the adult. The eggs are laid and the young hatched apparently within a period of from six to ten days.
Returning to the stage indicated by figures 166 and 167, I am induced to quote some remarks published in the Memoirs of the Peabody Academy of Science, No. 2, p. 18, which seem to support the view that these insects are offshoots from the Neuroptera.
"The front of the head is so entirely different from what it is in the adult, that certain points demand our attention. It is evident that at this period the development of the insect has gone on in all important particulars much as in other insects, especially the Neuropterous Mystacides as described by Zaddach. The head is longer vertically than horizontally, the frontal, or clypeal region is broad, and greater in extent than the epicranio-occipital region. The antennae are inserted high up on the head, next the ocelli, falling down over the clypeal region. The clypeus, however, is merged with the epicranium, and the usual suture between them does not appear distinctly in after life, though its place is seen in figure 167 to be indicated by a slight indentation. The labrum is distinctly defined by a well marked suture, and forms a squarish, k.n.o.b-like protuberance, and in size is quite large compared to the clypeus. From this time begins the process of degradation, when the insect a.s.sumes its Thysanurous characters, which consist in an approach to the form of the Myriopodous head, the front, or clypeal region being reduced to a minimum, and the antennae and eyes brought in closer proximity to the mouth than in any other insects."
Sir John Lubbock has given us an admirable account of the internal anatomy of these little creatures, his elaborate and patient dissections filling a great gap in our knowledge of their internal structure. The s.p.a.ce at our disposal only permits us to speak briefly of the respiratory system. Lubbock found a simple system of tracheae in Smynthurus which opens by "two spiracles in the head, opposite the insertion of the antennae," _i. e._, on the back of the head. (Von Olfers says that they open on the prothorax.) Nicolet and Olfers claim to have found tracheae in several lower genera (Orchesella, Tomocerus, and Achorutes and allied genera), but Lubbock was unable to detect them, and I may add that I have not yet been able after careful search to find them either in living specimens, or those rendered transparent by potash.
Having given a hasty sketch of the external aspect of the Poduras, I extract from Lubbock"s work a synopsis of the families and genera for the convenience of the student, adding the names of known American species, or indications of undescribed native forms.
SMYNTHURIDae.--Body globular or ovoid; thorax and abdomen forming one ma.s.s; head vertical or inclined; antennae of four or eight segments. Eyes eight on each side, on the top of the head. Legs long and slender.
Saltatory appendage with a supplementary segment.
Smynthurus. Antennae four-jointed, bent at the insertion of the fourth, which is nearly as long as the other three, and appears to consist of many small segments. No conspicuous dorsal tubercles. (In this country Fitch has described five species: S. arvalis, elegans, hortensis, Novaeboracensis, and signifer. Figure 156 represents a species found in Maine.)
Dicyrtoma. Antennae eight-jointed, five before, three after the bend. Two dorsal tubercles on the abdomen.
Papirius.[12] Antennae four-jointed, without a well-marked elbow, and with a short terminal segment offering the appearance of being many-jointed.
PODURIDae.--This family comprises those species of the old genus Podura, in which the mouth has mandibles [also maxillae and a labium], and the body is elongated, with a more or less developed saltatory appendage at the posterior extremity.
Orchesella. Segments of the body unequal in size, more or less thickly clothed with clubbed hairs. Antennae long, six-jointed. Eyes six in number on each side, arranged in the form of an S. (One or two beautiful species live about Salem.)
[Ill.u.s.tration: 168. Degeeria.]
Degeeria. Segments of the body unequal in size, more or less thickly clothed by clubbed hairs. Antennae longer than the head and thorax, filiform, four-jointed. Eyes eight in number on each side of the head.
(Two species, Degeeria decem-fasciata, Pl. 10, Figs. 2, 3, and D.
purpurascens, Figs. 4, 5, are figured in the "Guide to the Study of Insects." Figure 168 represents a species found in Salem, Ma.s.s., closely allied to the European D. nivalis. Five species are already known in New England.)
Seira. Body covered with scales. Antennae four-jointed; terminal segment not ringed. Eyes on a dark patch. Thorax not projecting over the head.
Abdominal segments unequal.
Templetonia. Segments of the body subequal, clothed by clubbed hairs, and provided with scales. Antennae longer than the head and thorax, five-jointed, with a small basal segment, and with the terminal portion ringed.
Isotoma. Four anterior abdominal segments subequal, two posterior ones small; body clothed with simple hairs and without scales. Antennae four-jointed, longer than the head; segments subequal. Eyes seven in number on each side, arranged in the form of an S. (Three species are found in Ma.s.sachusetts, one of which (I. plumbea) is figured on Pl. 10, Figs. 6, 7, of the "Guide to the Study of Insects," third edition.)
Tomocerus. Abdominal segments unequal, with simple hairs and scales.
Antennae very long, four-jointed, the two terminal segments ringed. Eyes seven in number on each side. (The European T. plumbea, Podura plumbea of authors, is our species, and is common. Fig. 160, greatly enlarged, copied from Templeton; Fig. 159, side view, see also Fig. 161, where the mouth-parts are greatly enlarged, the lettering being the same, _md_, mandibles; _mx_, maxillae; _mp_, maxillary palpus; _lb_, labium; _lp_, l.a.b.i.al palpus; _lc_, lacinia; _g_, portion ending in three teeth; _l_, lobe of labium; _sp_, ventral sucking disk; the dotted line"s pa.s.sing through the body represent the course of the intestine; _b_, end of tibia, showing the tarsus, with the claw, and two accessory spines; _a_, third joint of the spring. Fig. 162, lacinia of maxilla greatly enlarged. Fig. 169, different forms of scales, showing the great variation in size and form, the narrow ones running into a linear form, becoming hairs. The markings are also seen to vary, showing, their unreliable character as test objects, unless a single scale is kept for use.)
[Ill.u.s.tration: 169. Scales of Tomocerus.]
[Ill.u.s.tration: 170. Lepidocyrtus.]
[Ill.u.s.tration: 171. Scale of Lepidocyrtus.]
Lepidocyrtus. Abdominal segment unequal, with simple hairs and scales.
Antennae long, four-jointed. Eyes eight in number on each side. (Fig.
170, L. albinos, an European species, from Hardwicke"s "Science Gossip."
Fig. 171, a scale. Two species live in New England.)
Podura. Abdominal segments subequal. Hairs simple, no scales. Antennae four-jointed, shorter than the head. Eyes eight in number on each side.
Saltatory appendage of moderate length.
[Ill.u.s.tration: 172. Achorutes.]
Achorutes. Abdominal segments subequal. Antennae short, four-jointed.
Eyes eight in number on each side. Saltatory appendage quite short.
Figure 172 represents a species of this genus very abundant under the bark of trees, etc., in New England. It is of a blackish lead color; _a_, end of tibia bearing a tenant hair, with the tarsal joint and large claw; _b_, spring; _c_, the third joint of the spring, with the little spine at the base; figure 163, the supposed ovipositor; _a_, the two blades spread apart; _b_, side view. The mouth-parts in this genus are much as in Tomocerus, the maxillae ending in a lacinia and palpus.
[Ill.u.s.tration: 173. Lipura fimetaria.]
The three remaining genera, Lipura, Anurida and Anura, are placed in the "family" Lipuridae, which have no spring. Lubbock remarks that "this family contains as yet only two[13] genera, Lipura (Burmeister), in which the mouth is composed of the same parts as those in the preceding genera, and Anura (Gervais), in which the mandibles and maxillae disappear." Our common white Lipura is the European L. fimetaria Linn.
(Fig. 173, copied from Lubbock). The site of the spring is indicated by an oval scar.
[Ill.u.s.tration: 174. 176. 175.
Anurida maritima.]
Figure 174 represents Anurida maritima found under stones between tide marks at Nantucket. It is regarded the same as the European species by Lubbock, to whom I had sent specimens for comparison. This genus differs in the form of the head from Lipura and also wants the terminal upcurved spines, while the antennae are much more pointed. The legs (Fig. 175) end in a large, long, curved claw. On examining specimens soaked in potash, I have found that the mouth-parts of this species (Fig. 176,) _md_, mandibles; _mx_, maxillae; _e_, eyes, and a singular accessory group of small cells, are like those of Achorutes, as previously noticed by Laboulbene. The mandibles, like those of other Poduras, end in from three to six teeth, and have a broad, many-toothed molar surface below.
The maxillae; end in a tridentate lacinia as usual, though the palpi and galea I have not yet studied.
The genus Anura may be readily recognized by the mouth ending in an acutely conical beak, with its end quite free from the head and hanging down beneath it. The body is short and broad, much tuberculated, while the antennae are short and pointed, and the legs are much shorter than in Lipura, not reaching more than a third of their length beyond the body.
Our common form occurs under the bark of trees.
For the reason that I can find no valid characters for separating these three genera as a family from the other Poduras, I am inclined to think that they form, by the absence of the spring, only a subdivision (perhaps a subfamily) of the Poduridae.
The best way to collect Poduras is, on turning up the stick or stone on the under side of which they live, to place a vial over them, allowing them to leap into it; they may be incited to leap by pushing a needle under the vial. They may also be collected by a bottle with a sponge saturated with ether or chloroform. They may be kept alive for weeks by keeping moist slips of blotting paper in the vial. In this way I have kept specimens of Degeeria, Tomocerus and Orchesella, from the middle of December till late in January. During this time they occasionally moulted, and Tomocerus plumbeus, after shedding its skin, ate it within a few hours. Poduras feed ordinarily on vegetable matter, such as dead leaves and growing cryptogamic vegetation. These little creatures can be easily preserved in a mixture of alcohol and glycerine, or pure alcohol, though without the glycerine the colors fade.
We have entered more fully in this chapter into the details of structure than heretofore, too much so, perhaps, for the patience of our readers.
But the study of the Poduras possesses the liveliest interest, since these lowest of all the six-footed insects may have been among the earliest land animals, and hence to them we may look with more or less success for the primitive, ancestral forms of insect life.
FOOTNOTES:
[Footnote 9: Nicolet, in the "Annales de la Societe Entomologique de France" (tome v, 1847), has given us the most comprehensive essay on the group, though Latreille had previously published an important essay, "De l"Organization Exterieure des Thysanoures" in the "Nouvelles Annales du Museum d"Histoire Naturelle, Paris, 1832," which I have not seen.
Gervais has also given a useful account of them in the third volume of "Apteres" of Roret"s Suite a Buffion, published in 1844.
The Abbe Bourlet, Templeton, Westwood, and Haliday have published important papers on the Thysanura; and Meinert, a Danish naturalist, and Olfers, a German anatomist, have published important papers on the anatomy of the group. In this country Say and Fitch have described less than a dozen species, and the writer has described two American species of Campodea, C. Americana, our common form, and C. Cookei, discovered by Mr. C. Cooke in Mammoth Cave, while Humbert has described in a French scientific journal a species of Jupyx (J. Saussurii) from Mexico.]