1. Involution under the formation of a continuous dorsal amnion-serosa-sac (Odonata).
2. Involution with exclusively dorsal absorption of the amnion (Doryphora).
3. Involution with exclusively dorsal absorption of serosa and separation of the amnion (Chironomus and Trichoptera).
Fig. 532.—Diagram of the formation of the dorsal walls in Doryphora in cross-sections: am, amnion; in B, serving as a provisional dorsal closure, in C, about to break up; k, primitive band; s, serosa.—After Wheeler, from Korschelt and Heider.
4. Involution with separation of both embryonic membranes (Lepidoptera and Hymenoptera, Hylotoma).
Fig. 533.—Involution of the embryonic membranes of Chironomus: am, amnion; r, dorsal umbilicus; s, serosa, which has withdrawn into the region of the dorsal umbilicus, and in C has passed into the interior of the embryo.—After Graber, from Korschelt and Heider.
The first type occurs in the most primitive order of winged insects. The second type (Coleoptera) appears to be an independently inherited form of dorsal closure. In the first type, the formation of the amnion-serosa-sac is initiated by a rupture of the two fused embryonic membranes. This rupture in the ventral middle line occurs in Odonata only in the region of the head-section. In the second type only the amnion, in the third only the serosa are concerned in this rupture, while in the fourth type both membranes remain intact until the slipping out of the larva. (Korschelt and Heider.)