THOEACIC APPENDAGES. 67 



connected closely by a membrane with the scutellum above and loosely 

 with the flank below ; it is covered with fine hairs. The second joint of 

 the costal nervure is united to the first by membrane. It is an elongated 

 scale, with its anterior edge curved downwards, so that the anterior edge 

 of the wing appears to possess considerable thickness. The cavity of this 

 scale is very narrow, and, I believe, becomes obliterated entirely in the 

 adult insect. This scale bears a large number of short thick hairs. The 

 costal nervure is continued directly backwards along the margin of the wing 

 from the posterior extremity of this scale. There is, however, the indica- 

 cation of a joint between them. The uervure is set with strong spine-like 

 hairs, and its canal is entirely obliterated by the fibrillation of the 

 mesoderm. 



The sub-costal nervure, unlike the costal, remains tubular during the 

 whole life of the insect, and contains a nerve and tracheal vessel in their 

 passage to certain structures, which may be observed on the nervure. 



The sub-costal is almost cylindrical, except at its origin, where it 

 spreads out into a broad scale, which is closely united to the basal scale 

 of the costal nervure. A strong tooth projects from its underside near to 

 its origin, which I shall call the wing-pivot. This consists of a clear 

 amber- coloured, excessively hard process of chitine, exactly similar to 

 that which forms the extremity of the mandibles in some insects. Its 

 extremity fits into a hollow, in a similar process, which is supported by 

 the flank, and which is shaped something like an inverted molar tooth 

 (Plate V., figs. 12 and 14). 



The organs on the sub-costal nervure, to which the nerve is distributed, 

 are several groups of transparent spots, exactly similar to those on the 

 base of the halteres. These were first pointed out by Dr. Braxton Hicks,* 

 and appear very couhtant in all insects except the Orthoptera and the 

 Coir opt era, where their presence needs confirmation, as they are very 

 different from the double refracting circular bodies so common in their 

 elytra, with which they appear, however, to have been confounded. These 

 organs will be described at length in Section XVIII. I have also traced 

 filaments of the nerve to the bulbs of the hairs, which are situated on an 

 enlargement of the posterior branch of the sub-costal. 



Behind the origin of the sub-costal nervure is a thickened chitinous 

 fold, which bears the winglets (plate V., fig. 126). It is closely connected 

 above with the sub-costal nervure, and beneath with the rod which forms 

 the interior margin of the scutellum. Its external process bears the 

 winglets which are attached to its margin. When the wing is at rest, this 



* Linn. Soc. Trans., 1856-57. 



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