ANATOMY AND PHYSIOLOGY 



6l 



the base of the abdomen in relation to the thorax, but especially to 

 modifications of the extremity of the abdomen, for sexual purposes. 



Modifications. In aculeate Hymenoptera the first segment of the 

 abdomen has been transferred to the thorax, where it is known as the 

 propodeum, or median segment; in other words, what appears to be the 

 first abdominal segment is actually the second; this, as in bees and 

 wasps, often forms a petiole, which enables 

 the sting to be applied in almost any direc- 

 tion. In Cynipidae the tergum of segment 

 two or three occupies most of the abdom- 

 inal mass, the remaining segments being 

 reduced and inconspicuous. The terminal 

 segments of the abdomen often telescope 

 into one another, as in many Coleoptera and 

 Hymenoptera (Chrysididae), or undergo 

 other modifications of form and position 

 which obscure the segmentation. As to 

 the number of evident (not actual) abdom- 

 inal segments, Coleoptera show five or six 

 ventrally and seven or eight dorsally; 

 Lepidoptera, seven in the female and eight 

 in the male; Diptera, nine (male Tipulidae) 

 or only four or five; and Hymenoptera, nine 

 (Tenthredinidae) or as few as three (Chry- 

 sididae). In the larvae of these insects, 

 nine or ten abdominal segments are usually 

 distinguishable, though the tenth is fre- 

 quently modified, being in caterpillars 



United with the ninth. FIG. 78. Ventral aspect of the 



j -r- i i abdomen of a female Machilis 



Appendages. Rudimentary abdominal m0 r.lm, to show rudimentary 



limbs occur in Thysanura (Machilis, Fig. 



78) . Functional abdominal legs do not OCCUr segment is omitted.) c, c, c, lateral 

 ji,. i_ .L i J.T. T_J cerci and median pseudocercus. 



in adult insects, but in larvae the abdom- MteT QUDEMANS. 

 inal rolegs (Fig. 66) are homologous 



with the thoracic legs and the other paired segmental appendages, as 

 the embryology shows. The embryo of (Ecanthus, according to Ayers, 

 has ten pairs of abdominal appendages (Fig. 199), equivalent to the 

 thoracic legs. Most of these embryonic abdominal appendages are 

 only transitory, but the last three pairs frequently persist to form the 

 genitalia, as in Orthoptera (to which order (Ecanthus belongs). In 



