62 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



THE SUBSEROSA 



About the time the embryo of the snout beetle Brachyrhinus ligustici 

 has formed, a substance appears just under the serosa that gradually 

 thickens and hardens until it forms a brown leathery shell around the 

 contents of the egg (Fig. 255). Because of its position during the greater 

 part of its existence Butt (1936) has called it the "subserosa." It is 

 noncellular, nongranular and stains a light brown when haemotoxjdin 

 and orange G are used and a deep blue with Mallory's connective- 

 tissue stain. When it first appears as a definite layer next to the serosa 

 at approximately 40 hours, it is not very thick but lies on the inner surface 

 of the serosa cells and extends down between the cell walls. The serosa 

 cells are still columnar in form, as this is the last part of the blastoderm 

 to differentiate. From the appearance of the serosa cells at this time it 

 seems evident that the subserosal substance is secreted by them and 

 possibly also by the cells of the amnion and the cells forming the embryo. 

 Butt suggests that the substance has a protective function. From a 

 figure given by Scheinert (1933) of a snout beetle, Liparus germanus, it is 

 evident that the egg of this species is provided with a similar protective 

 coat. 



BLASTOKINESIS 



By this term Wheeler (1893) designated all the oscillatory movements 

 or flections of the germ band during development. The ascending stage 

 he called " anatrepsis " ; the descending, " katatrepsis " ; the intervening 

 resting stage, the "diapause" (Figs. 36, 37). Since the flections of the 

 embryo in some cases cannot be sharply distinguished from shifts due to 

 growth in length and to later contraction, the expression is now usually 

 used for all displacements, rotations, or revolutions of the embryo within 

 the egg. 



In its broadest sense, blastokinesis then may involve the growth in 

 length of the germ band, the caudal end growing backward, pushing 

 either into the yolk or over the dorsal side, in many cases the embryo 

 becoming reversed in position with the head directed caudad. The 

 diapause then ensues, followed in many instances by rupture of the 

 envelopes and a subsequent reversal of the embryo to its original position 

 with the head again directed forward. In this case the embryo under- 

 goes a revolution about an axis perpendicular to the sagittal plane of 

 the embryo. This occurs in general in the Orthoptera, Dermaptera, 

 Odonata, Ephemerida, Heteroptera, Homoptera, Anoplura, Mallophaga, 

 etc. In other cases the rotation is about the longitudinal axis of the 

 embryo, as with some Lepidoptera (Diacrisia), nemocerous Diptera 

 {Chironomus, Simulium), Melanoplus (Slifer, 1933), and the mantid 



