NO. 2 HONEY BEE — SNODGRASS 3 



sclerites isolated in otherwise membranous areas are themselves func- 

 tional units. Most sclerites, therefore, instead of being primitive 

 elements of the exoskeleton, are more probably recent developments 

 accompanying the evolution of skeletal mechanisms. 



With regard to the homology of sclerotized areas the question 

 often arises as to whether or not an area of sclerotization represents 

 necessarily a primary morphological area. As, for example, if a so- 

 called pleural plate that ordinarily is confined to the side of the 

 thorax is found in some species to be continued down upon the 

 venter, are we to assume that the original pleural integument has 

 accompanied the sclerotic extension into the venter at the expense 

 of the primitive sternal integument, or is it merely that the physio- 

 logical process of cuticular sclerotization has spread beyond the usual 

 limits? Until this question can be answered, confusing problems in 

 nomenclature will always intrude themselves into practical anatomical 

 descriptions. 



A similar question concerns the significance or morphological value 

 of muscle attachments. Do muscle attachments necessarily imply 

 homology in the skeletal parts, or can muscles shift their attachments 

 from one place to another? In answer to this question there is 

 abundant evidence that the so-called insertion points of muscles 

 seldom show radical changes, while, on the other hand, the points 

 of origin may be at widely different places on the skeleton in different 

 species. Evidence of homology, therefore, is pretty sound when 

 based on the working end of the muscle, though the work performed 

 by the muscle may be entirely different in two species by reason of a 

 change in the position of the muscle or some alteration in the skeletal 

 mechanism. We do not know, however, whether a muscle arising at 

 different places in two species has migrated during the evolution of 

 one species or the other, or whether a muscle, during its formative 

 stage of development, may evoke an entirely new attachment on the 

 body wall by some hormonal effect at the point where its end comes 

 in contact with the epidermis. 



The musculature of the head and the feeding organs appears to be 

 more nearly standardized than that of any other part of the insect, 

 regardless of the great variation in structure and function of the 

 mouth parts. The muscles of the thorax, the legs, the abdomen, and 

 the ovipositor conform fairly well with an underlying plan, but the 

 number of muscles is far from constant. The musculature of the 

 male genitaHa, however, shows no prevailing pattern and may be 

 entirely different in different orders of insects. The contrast between 

 the stability of the mouth part musculature and the variable nature 



