rackard.] 282 



and Dr. B. G. Wilder, Involving a repetition of homologous append- 

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

 appendage to the tenth ring in Agrion, seems homologous, both In func- 

 tion and structure, with the respiratory lamella? of the swimming 

 abdominal limbs of the lower decapodous Crustacea and the tetradeca- 

 pods, 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 hymenoptera. This we 

 deem the most essential zoological character separating the hymenop- 

 tera 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, peculiar to the higher families of the hymenop- 

 tera ; as In the lowest, the Tenthredinidae, the transition Is but par- 

 tial, corresponding to the Lepldoptera in this respect. It Is an 

 instance of the principle of cephalization advanced by Professor 

 Dana, so fully illustrated in the Crustacea, where occur in some 

 groups changes In the primitive number of arthromeres, proved 

 by the inconstant number of rings (arthromeres) forming the abdo- 

 men, and cephalo-thorax respectively. This transfer of the zoologi- 

 cal 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 classification 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 propocleum. But our 

 best hymenopterists of thirty years' standing consider it to be the meta- 

 thorax, with the exception of Baron Osten Sacken In his articles on 

 the Cynipida3.* During the autumn of 1863, when the observations 

 here recorded were made, our attention was drawn f to this part. At 

 this period the thorax is one-third smaller than in the pupa. The 

 position of the three thoracic spiracles can be easily discerned. On 

 the two posterior rings of the thorax they are seen situated In their 

 respective "peritremes" (Audouin), which pieces lie at the base, and 



* Proceedings of the Entomological Society of Philadelphia. Vols. IT, III. 

 t Proceedings Essex Institute. Vol. IV. The Humble Bees of New England and 

 their parasites; etc. Communicated April 23, 1864. p. 3. Kote. 



