40 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



macrosomitic segmentation sets in only after the germ band has split up 

 into about eight segments, and he therefore concluded that, as opposed 

 to the view of Graber (1890), macrosomitic segmentation has no phylo- 

 genetic significance. 



In primitive insects 12 abdominal segments may be present, the 

 terminal anus-bearing segment without appendages being the twelfth. 

 In malacostracan Crustacea this terminal segment, or periproct, is the 

 telson. Insects in which a twelfth segment has been described are, 



pro.c 



ptc 





r: 



■Ir 



ant 

 ani2 



1=1- 



head 



fh 



^ 



abcl{ 



\/li 





^.- 4 



Fig. 28. Fig. 29. 



Figs. 28, 29. — Two stages of embryos representing two periods in evolutionary history 



of insects, {abd) Abdomen, (ant) Antenna, (ant 2) Second antenna. (lb) Labium. 



{Ir) Labrum. (md) Mandible, {mx) Maxilla, (p) legs, (pleur) Pleuropodium. (pro.c) 



Protocephalon. (pic) Protocorm. (siom) Stomodaeum. (th) Thorax. {From Snodgrass.) 



among others, Lepisma, Gryllotalpa, Eutermes, and Chalicodoma. In the 

 embryos of most Pterygota, however, 11 represents the maximum 

 number, as found in Locusta (Roonwal), Calandra (Tiegs), Diacrisia 

 (Johannsen), Siphonaptera (Kessel), Apis (Nelson), etc. 



The question as to the number of segments that enter into the 

 composition of the head of an insect or indeed of an arthropod generally 

 has been the subject of much controversy. Since the segments of the 

 gnathal (jaw) region are distinct in the embryo, the question centers on 

 the number of segments involved in the procephalic region of insects. 

 Until recently workers have also been in general agreement as to an 

 antennal and an intercalary segment lying immediately in front of the 



