In certain Dragon-flies (Calepteryx), and Hemiptera (Hydrometra), the legs, according to Brandt,[20] appear at a still earlier stage.
According to the observations of Kölliker,[21] it would appear that in the Coleopterous genus Donacia the segments and appendages appear simultaneously.
Kölliker himself, however, frankly admits that “meæ de hoc insecto observationes satis sunt manca,” and it is possible that he may never have met with an embryo in the state immediately preceding the appearance of the legs; especially as it appears from the observations of Kowalevski that in Hydrophilus the appendages do not make their appearance until after the segments.[22]
On the whole, as far as we can judge from the observations as yet recorded, it seems that in Homomorphous insects the ventral wall is developed and divided into segments, before the appearance of the legs; but that the latter are formed almost simultaneously with the cephalic appendages, and before either the dorsal walls of the body or the internal organs.
Fig. 32.—Egg of Pholcus opilionides (after Claparède).
As it is interesting, from this point of view, to compare the development of other Articulata with that of insects, I give a figure (Fig. [32]), representing an early stage in the development of a spider (Pholcus) after Claparède,[23] who says, “C’est à ce moment qu’a lieu la formation des protozonites ou segments primordiaux du corps de l’embryon. Le rudiment ventral s'épaissit suivant six zônes disposées transversalement entre le capuchon anal et le capuchon céphalique.”