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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



eralized insects. Heymon's general conclusion is that the pleural parts 

 of adult insects may not correspond exactly in their entire extent with 

 the subcoxal leg segment of the embryo, but that they are either in 

 part or principally derived from it. The subject needs further sup- 

 port from embryology ; but the subcoxal theory is now generally ac- 

 cepted in one form or another by most European students of insect 

 morphology, though some follow Hansen in regarding the trochantin 

 alone as representing the subcoxa. 



Fig. i6. — Legs and pleural parts of immature insects, suggesting that the 

 pleural sclerites belong to a basal, subcoxal segment of the leg (Sex). 



A, middle leg and pleural sclerites of larva of Scarites (Carabidse). 



B, middle leg and pleural sclerites of larva of Pteronidea ribesi (Tenthredin- 

 idae). 



C, abdominal leg of a caterpillar (Lepidoptera). 



D, hind leg and pleuron of mature nymph of Tibicina septendecim (Homop- 

 tera). 



Students of other groups of Arthropoda also recognize a subcoxal 

 segment, or plciiropoditc, as the true base of the primitive limb 

 (fig. 42 A). Some of the Acarina furnish particularly suggestive 

 examples of structures that appear to be subcoxal leg segments. In 

 the ticks (Ixoidea) each leg is articulated to a large basal piece 

 (fig. 17, Sex) expanded on the ventral surface of the body, but con- 

 tinued over the base of the coxa as a narrow arch in the pleural 

 surface. These leg bases are provided with tergal muscles (Derma - 

 centor, Ainblyomnia), and those of the first pair at least are capable 

 of a slight rotary motion on their transverse axes. Each, moreover. 



