and Position of the Hymenoptera. 85 



arising as they do from the arthropleural or limb- bearing region 

 of the body, i. e. between the sternum and episternum (or lower 

 pleurite), are strictly homologous with the abdominal legs of the 

 Myriapoda and the "false legs" of caterpillars; so that in 

 these genito-sensory appendages we perceive faint traces of the 

 idea of antero-posterior symmetry first observed in Vertebrates 

 by Oken, and more recently by Professor Wyman and Dr. 13. G. 

 Wilder, involving a repetition of homologous appendages at the 

 two opposite poles of the body. The broad leaf-like appendage 

 to the tenth ring in Agrion seems homologous, both in function 

 and structure, with the respiratory lamellse of the swimming 

 abdominal limbs of the lower decapodous Crustacea and the 

 Tetradecapods, which perform the function of gills. 



During this stage, the basal ring of the abdomen of Bombus 

 (fig. 2 c) is plainly seen to be transferred from the abdomen to 

 the thorax, with which it is intimately united in the Hymeno- 

 ptera. This we deem the most essential zoological character 

 separating the Hymenoptera from all other insects. This transfer 

 of an entire arthromere from one region to that next in front, 

 involving the remodelling of the entire f(^rm of the insect, though 

 not uncommon in the Crustacea, is, in the class of Insects, pe- 

 culiar to the higher families of the Hymenoptera, as in the 

 lowest (the Tenthredinidse) the transition is but partial, corre- 

 sponding to the Lepidoptera in this respect. It is an instance 

 of the principle of cephalization advanced by Professor Dana, so 

 fully illustrated in the Crustacea, where in some groups changes 

 occur in the primitive number of arthromeres, proved by the 

 inconstant number of rings (arthromeres) forming the abdomen 

 and cephalothorax respectively. This transfer of the zoological 

 elements from the posterior end of an animal towards the head, 

 involving in this act the entire I'cconstruction of the animal form, 

 lies at the basis of all sound classification, and is a principle 

 which must be followed by every student dealing with the clas- 

 sification of the larger divisions of the animal kingdom. 



So intimately united with the thorax is this elemental ring, 

 that, from its sculpturing, its coloration, and, in fine, its close 

 mimicry of the normal thoracic segments, our best observers 

 have nnited in calling it the metathorax, and homologizing it 

 with that ring in the lower insects. Latreille and Audouin 

 considered it as the basal ring of the abdomen, as did Newman, 

 who termed it the propodeum. But our best hymenopterists of 

 thirty years' standing consider it to be the metathorax, with 

 the exception of Baron Osten Sacken, in his articles on the 

 Cynipidse*^ During the autumn of 1863, when the observa- 



* Proceedings of the Entomological Society of Philadelphia, vols, ii., iii. 



