and Position of the Hymenoptera. 85 
arising as they do from the arthropleural or limb-bearing region 
of the body, z. 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. B. 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 lamelle 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 form 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 Tenthredinidz) 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 reconstruction 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 united 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 
Cynipide*. During the autumn of 1863, when the observa- 
* Proceedings of the Entomological Society of Philadelphia, vols. ii., iii. 
