Fig. 180. Two stages in the development of Hydrophilus piceus. (From Gegenbaur, after Kowalevsky.)
ls. labrum; at. antenna; md. mandible; mx. maxilla I.; li. maxilla II.; p´ p´´ p´´´. feet; a. anus.
The appendages appear as very small rudiments at the close of the last stage, but soon become much more prominent ([fig. 180] A). They are formed as outgrowths of both layers, and arise nearly simultaneously. There are in all eight pairs of appendages. The anterior or antennæ (at) spring from the procephalic lobes, and the succeeding appendages from the segments following. The last pair of embryonic appendages, which disappears very early, is formed behind the third pair of the future thoracic limbs. Paired epiblastic involutions, shewn as pits in the posterior segments in [fig. 180] A, give rise to the tracheæ; and the nervous system is formed as two lateral epiblastic thickenings, one on each side of the mid-ventral line. These eventually become split off from the skin; while between them there passes in a median invagination of the skin ([fig. 189] C). The two nervous strands are continuous in front with the supra-œsophageal ganglia, which are formed of the epiblast of the procephalic lobes. These plates gradually grow round the dorsal side of the embryo, and there is formed immediately behind them an oral invagination, in front of which there appears an upper lip ([fig. 180], ls). A proctodæum is formed at the hind end of the body slightly later than the stomodæum. The mesoblast cells become divided into two bands, one on each side of the middle line ([fig. 189] A), and split into splanchnic and somatic layers. The central yolk mass at about the stage represented in [fig. 179] begins to break up into yolk spheres. The hypoblast is formed first on the ventral side at the junction of the mesoblast and the yolk, and gradually extends and forms a complete sack-like mesenteron, enveloping the yolk ([fig. 185] al). The amnion and serous membrane retain their primitive constitution for some time, but gradually become thinner on the ventral surface, where a rupture appears eventually to take place. The greater part of them disappears, but in the closure of the dorsal parietes the serous envelope plays a peculiar part, which is not yet understood. It is described on p. [404]. The heart is formed from the mesoblastic layers, where they meet in the middle dorsal line ([fig. 185] C, ht). The somatic mesoblast gives rise to the muscles and connective tissue, and the splanchnic mesoblast to the muscular part of the wall of the alimentary tract, which accompanies the hypoblast in its growth round the yolk. The proctodæum forms the rectum and Malpighian bodies[169], and the stomodæum the œsophagus and proventriculus. The two epiblastic sections of the alimentary tract are eventually placed in communication with the mesenteron.
The development of Hydrophilus is a fair type of that of Insects generally, but it is necessary to follow with somewhat greater detail the comparative history of the various parts which have been briefly described for this type.
The embryonic membranes and the formation of the layers.
All Insects have at the close of segmentation a blastoderm formed of a single row of cells enclosing a central yolk mass, which usually contains nuclei, and in the Poduridæ is divided up in the ordinary segmentation into distinct yolk cells. The first definite structure formed is a thickening of the blastoderm, which forms a ventral plate.
The ventral plate is very differently situated in relation to the yolk in different types. In most Diptera, Hymenoptera and (?) Neuroptera (Phryganea) it forms from the first a thickening extending over nearly the whole ventral surface of the ovum, and in many cases extends in its subsequent growth not only over the whole ventral surface, but over a considerable part of the apparent dorsal surface as well (Chironomus, Simulia, Gryllotalpa, etc.). In Coleoptera, so far as is known, it commences as a less extended thickening either of the central part (Donacia) or posterior part (Hydrophilus) of the ventral surface, and gradually grows in both directions, passing over to the dorsal surface behind.
Embryonic membranes. In the majority of Insects there are developed enveloping membranes like those of Hydrophilus.
