Fossorial Hymenoptera of North America. 43 



holes in posts as Crabro swgularis does, according to the observations 

 of Mr. Shurtleff, and as I have seen done by Philanthus, thus adapting 

 them to the requirements of their young; or like the Rhopalvm pedi- 

 ccllatum, Stigmus fraternut and Crabro stripico7a, avail themselves 

 of those plants wbose stem lias a pith which they can readily excavate 

 and refit for their habitations. Thus the great variability of the fore 

 legs in fossorial hymenoptera is especially marked; in the digging and 

 tunnelling Thyreopus with its broad dilations and abnormal enlarge- 

 ment of the fore legs we are reminded strikingly of the moles among 

 vertebrata. In the Apidae, the hind legs are especially used for 

 gathering pollen, while the fore legs assist the mouth parts in build- 

 ing and elaborating the cells and ucst. In this family is a greater 

 differentiation in the structure of the legs, correlated with the more 

 diverse uses of the limbs. As we descend in the hymenoptera we find 

 a great equality in form in the legs of the Ichueuuionida: and Tenthre- 

 dinidse; in some Chalcids, however, which in most respects are higher 

 than the IchneumonidcB, the hind legs are greatly enlarged for leaping 

 like the coleopterous Halticus. Other cases will readily suggest them- 

 selves to entomologists. 



In classifying the genera, within a sub-family, as well as the larger 

 groups, the principle of Cephalization as advanced by Prof. Dana* has 

 always been our chief guide in arranging the hymenoptera, and before 

 meeting with the views advanced by that author who has thrown so 

 much light on the study of the articulates, we were accustomed to place 

 highest those members of a group whose bodies were most concentrated, 

 and had the elements of organization thrown farthest towards the head. 



O 



In illustration, we regard Halictus as lower than And rata or Apis ; 

 Zethus and Eumenes as holding a subordinate position to Polistes and 

 Vespa ; in the Crabronidae, Stigmus as inferior to Blepharrpus, Psen 

 to C< rceris ; in the Nyssonidte, Trypoxylon as much lower than Oxi/behis, 

 and so down the scale. In all these lower forms, we see the body 

 lengthened out, the rings of the body more equally developed, showing 

 a decided tendency to " vegetative repetition" so marked in the worms, — 

 the "weight of organization" is withheld from the thorax and head, and 

 retained in the abdomen. In short, the insect is degraded, decephalized. 

 On the contrary, in what we believe to be the highest hymenopterous 

 insect, the Honey bee, the parts of the body are all more referred to the 

 head, thrown forwards and subordinated to that portion of the nervous 

 system residing in the head, which is more analogous to the brain of the 



"*'See several articles in the Amer. Journ. Sc. and Arts, 1S63 — 1866. 



