In Blatta, Hydrophilus, the Trichoptera, and the Lepidoptera the hindermost part of the primitive band is turned in ventrally (Figs. 534, C).
The preceding account of the relations of the primitive band to the yolk does not apply to all insects, since there are variations which appear to depend on the form of the egg, and on the amount and distribution of the yolk-masses. In certain Coleoptera, the primitive band sinks down and thus becomes immersed into the yolk. In Donacia (Kölliker and Melnikow) and Hydrophilus (Heider), and in the Chrysomelidæ and Attelabus, a weevil, as we have observed, the primitive band rests on the outside of the yolk, but in Telephorus fraxini it is immersed. In the Hemiptera it is immersed (Fig. 516), but there is much variation in this respect, the degree of immersion being most marked in the Coccidæ (Aspidiotus), and least so in Corixa. Besides the position of the primitive band, there are in Odonata and Hemiptera differences in the origin of the primitive band itself and of the embryonic membranes.
Fig. 515.—Ventral view of five developmental stages of Hydrophilus: a and b, places at which the blastopore contracts; af, edge of the amnion-fold; af′, caudal fold; af″, paired head-fold of the amnion; an, antenna; es, last segment; g, pit-like invagination (first indication of the amniotic cavity); k, head-lobes; r, furrow-like invagination; s, portion of the primitive streak covered by the amnion.—After Heider, from Lang.
Fig. 516.—Embryo of the louse: am, serosa; db, amnion; as, antenna; vk, clypeus.—After Melnikow.
Korschelt and Heider divide the early embryo of insects into two types:
1. Into those with a superficial primitive band; viz., where there is no passage of yolk-elements into the space between the amnion and serosa. The primitive band has in such cases a relatively superficial position (Figs. 508, 509, 521, 535). Examples are certain Orthoptera (Blatta, Œcanthus, Mantis, Gryllotalpa), also certain Hemiptera (Corixa), certain Coleoptera, and the Trichoptera, Diptera, and Hymenoptera.
2. Into those with an immersed primitive band, with the space between the serosa and amnion filled with yolk (Figs. 517, 518, 534). Examples are the orthopterous Stenobothrus, Odonata, many Hemiptera (the Pediculina and Pyrrhocoris), the Coleoptera already mentioned, and Lepidoptera.
It should be observed, however, that these differences are of little phylogenetic or taxonomic value, since genera of the same order, notably the Coleoptera, differ as to the position of the primitive band, so also two orders so nearly allied as the Trichoptera and Lepidoptera.