1921] Wheeler: Some Social Beetles 113 



its cavity and the histological structure of its walls, and especially 

 to the singular arrangement of the nutritive parenchyma in long, 

 narrow strands. The beetles gnaw these out and thus form 

 grooves, which in turn orient the coccids and their internal para- 

 sites. The orientation of the frass ridges and beetle cocoons is 

 also determined by the grooves, and the position of the Coccido- 

 trophus eggs is very probably due to their being laid and attached 

 by the beetles while they are lying in the depressions between 

 the frass ridges. Thus the various orientations are merely so 

 many direct or indirect adaptations to the nutritive parenchyma 

 and the long narrow petiolar cavity. The latter clearly deter- 

 mines to some extent the longitudinal arrangement of the bulky 

 cocoons, just as a long Pullman car makes it advisable for us 

 to arrange the berths in a similar manner. 



One orientation, that of the entrance, is not so easily 

 explained. In all the petioles I have examined, the long axis of 

 the entrance is parallel with the long axis of the petiole. It is 

 evident that it precisely fits the head of the beetle and that the 

 latter while gnawing it must stand on the outer surface of the 

 petiole at right angles to its long axis. When the surface is 

 longitudinally grooved, as is sometimes the case, the entrance can, 

 of course, have no other orientation, but often the surface is 

 quite smooth and it is difficult to see why it should be easier for 

 the insect to gnaw through the tissue lengthwise rather than 

 crosswise of the grain. It is also evident that while guarding 

 the entrance the beetle has its long axis at right angles to the 

 long axis of the petiole, but since a tubular wall is built around 

 the inside of the orifice (Fig.ll) the insect's reaction might be 

 regarded as thigmotropic, like its reactions to the walls of the 

 food grooves. 



Nevertheless, I am convinced that the responses of the adult 

 beetle to contact stimuli may be more properly interpreted as 

 typical and highly adaptive reflexes. This is clearly indicated 

 by the structure of the insect. The long, parallel-sided, sub- 

 cylindrical form of the body and the shortness of the legs are 

 merely so many adaptations to living in narrow tubular cavities. 

 The same type of structure reappears as an independent develop- 

 ment in each of many different families of beetles, which live 

 in cylindrical cavities or burrows, e.g., the Scolytidse, Platypodi- 



