ANATOMY AND PHYSIOLOGY 



53 



a 



segmentation. As to the number of evident (not actual) abdominal seg- 

 ments, 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 (Tenthre- 

 dinidae) or as few as three (Chrysididae) . In the larvae of these insects, 

 however, nine or ten abdominal segments are usually distinguishable, 

 though the tenth is frequently modified, 

 being in caterpillars united with the ninth. 



Appendages. Rudimentary abdom- 

 inal limbs occur in Thysanura (Machilis, 

 Fig. 76). Functional abdominal legs do 

 not occur in adult insects, but in larvae the 

 abdominal pro-legs (often called " false legs." 

 Fig. 64) are homologous with the thoracic 

 legs and the other paired segmental appen- 

 dages, as the embryology shows. The em- 

 bryo of (Ecanthus, according to Ayers, has 

 ten pairs of abdominal appendages (Fig. 

 197), 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 Collembola, the embryo has 

 paired abdominal limbs, and those of the 

 first abdominal segment eventually unite 

 to form the peculiar ventral tube (Fig. 12) 

 of these insects, while those of the fourth 

 segment form the characteristic leaping 

 organ, orfurcula. 



Cerci. In many of the more generalized 

 insects the abdomen bears at its extremity 

 two or three appendages termed cerci. These 



occur in both sexes and are frequently long and multiarticulate, as in Thy- 

 sanura (Figs. 76, 9, 10) and Ephemerida (Figs. 19, B.; 84), though shorter 

 in cockroaches and reduced to a single sclerite in Acridiidae (Fig. 87) . The 

 paired cerci, or cercopoda of Packard, are usually though not always as- 

 sociated with the tenth abdominal segment and are homologous with 

 legs, as Ayers has found in (Ecanthus and Wheeler in Xiphidium. As to 

 their function, the cerci of Thysanura are tactile, and those of the cock- 



FIG. 76. Ventral aspect of the 

 abdomen of a female Machilis 

 maritima, to show rudimentary 

 limbs (a) of segments two to nine. 

 (The left appendage of the eighth 

 segment is omitted.) c, c, c, cerci. 

 After OUDEMANS. 



