22 



SOME ASPECTS OF MORPHOLOGY 



limbs in any class other than the two aforementioned cannot be 

 accepted as proven. In view of the remoteness of affinity between 

 insects and Crustacea, the structural differences in the appendages 

 of the two classes are very marked. Since it has long been 

 customary to regard the crustacean biramous limb as the standard 

 for comparison, discussion of arthropodan 

 limb-morphology usually has reference to 

 that type of appendage. While advantage 

 may be derived from a uniform standard 

 for comparison, it involves homologies which 

 require due precaution before being accepted 

 as being proven. 



A complete biramous appendage (Fig. 10) 

 consists of a basal region or protopodite 

 which bears the main shaft or endopodite 

 and a usually smaller shaft or exopodite. 

 The protopodite is divisible into a basal 

 sclerite or precoxa, an intermediate sclerite 

 or coxa, together with a distal element or 

 basis. The endopodite consists of six clearly 

 defined segments, each of which has received 

 an individual name, viz., preischium, ischium, 

 merus, carpus, propodus and dactylus (Hansen, 

 1930). The exopodite is composed of a 

 variable number of divisions or sub-seg- 

 ments, while the coxa often bears an outer 

 lobe or exite (commonly termed an epipodite). 

 Both coxa and basis may ea*"!! develop an 

 inwardly directed masticatory process or 

 eiidite (often known as a gnathohase). 

 The recent works of Hansen (1930), Snodgrass (1935) and of 

 Ewing (1928) have contributed to the better understanding of the 

 morphology of insect appendages. Much confusion has prevailed 

 in the past owing to the neglect of evidence afforded by the 

 musculature, with the result that there has been a great diversity 

 of opinion with respect to the homologies of many of the parts 

 concerned. The extensive researches of Hansen form a valuable 



Fig. 10. Diagram of a 

 complete biramous 

 crustacean appen- 

 dage, p, precoxa ; c, 

 coxa ; 6, basis ; i, 

 preischium ; i' , is- 

 chium; m, merus ; 

 ca, carpus ; pr, pro- 

 podus ; d, dactylus ; 

 en, en, endites ; ep, 

 epipodite ; ex, exo- 

 podite. 



