Our Common Insects / A Popular Account of the Insects of Our Fields, Forests, Gardens and Houses

by A. S. Packard

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in Smynthurus and Degeeria, and shortest in Achorutes (Fig. 172, b), where it forms a simple, forked tubercle; and is obsolete in Lipura and Anura, its place being indicated by an oval scar.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.

163. Catch holding spring of Achorutes.

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 homologous 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 Phryganeidæ and suggests on embryological grounds the near relationship of the Thysanura to the Neuroptera.

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, antennæ; 2, mandibles; 3, maxillæ; 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-shell 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 assumes 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.