NO. 6 INSECT ABDOMEN — SNODGRASS 5 



orders by the muscles inserted on them, which take their origins in 

 the supporting basal plates. 



11. The styli of the male gonopods become the movable claspers 

 known as the harpcs in the copulatory apparatus of holometabolous 

 insects. They are to be identified by their muscles which arise in the 

 supporting basal plates. The harpes may be divided each into a i)air 

 of movable claspers. 



12. Numerous accessory appendicular lobes and processes may be 

 developed on all parts of the male genital segment and on segments 

 associated with it. These organs are secondary and are not necessarily 

 homologous in the several orders ; they are often flexible at their bases, 

 but are to be distinguished from the true harpes and from the para- 

 meres, with which they are associated, by their lack of muscles. 



13. The postpedes, present in holometabolous larvae of several 

 orders, are the pygopods, or appendages of the tenth somite. The 

 postpedes are probably transformed into the appendicular processes 

 of adult males known as socii, found in adult Trichoptera and 

 Lepidoptera, or into the cercus-like appendages of adult chalasto- 

 gastrous Hymenoptera. 



14. The cerci are the uropods, or the appendages of the eleventh 

 somite. Typically each is situated in a membranous area laterad ot the 

 base of the epiproct, and above the paraproct. Muscles that move 

 the cercus arise on the tenth tergum. or also on the epiproct, but these 

 muscles are not necessarily primitive muscles of the cereal appendages. 

 There is no intrinsic evidence that the cerci have any genetic relation 

 with the paraprocts. It is doubtful if true cerci occur in any holo- 

 metabolous insects, except possibly in females of Mecoptera. 



It will be evident from statements given above, and more flagrantly 

 apparent in discussion to follow, that the writer still gives much 

 importance to the value of muscles as determinants of skeletal homol- 

 ogies — and this in the face of the edict against such practices recently 

 put forth by TI. J. Hansen (1930). However, there surely can be 

 no question that in studying the insect skeleton we are dealing with 

 the passive elements of mechanisms, in which the active parts are the 

 muscles. The principal sclerotic areas of the body segments, and of 

 the limb segments, are always directly or indirectly associated with 

 muscle attachments or with mechanical strains resulting from muscle 

 actions, and there is every reason for believing that scierites have 

 been correlated with muscles in their evolution, if not necessarily in 

 their origin. It is, of course, true that, just as some scierites are 

 secondary productions, so undoubtedly are some muscles. We must 

 admit that all kinds of deviations from a rule are possible ; but a few 



