No. 3-] BITTACOMORPHA CLAVIPES FABR, 159 



In the legs of normal Tipnlidac (Pachyrrhina sp. ? and sev- 

 eral other species) the trachea occupies a considerable space 

 only in the femur and tibia, where it fills up from one-fourth 

 to one-sixteenth of the cavity. In the tarsus the tracheal tube 

 is very delicate or obsolete. 



The female of the peculiar parasitic hymenopteron, Pele- 

 cinus polyturator Drury, presents an external appearance simi- 

 lar to that of Bittacomorpha in the enlarged hind tibia (Fig. 6). 

 Here, however, the chitin of the external wall is thick and 

 heavy, and the trachea is robust, strongly striated, but not at 

 all dilated. 



It is the rule in insects, wherever a tracheal dilation occurs, 

 that the taenidiabecome obsolete, but thethinning of the tracheal 

 wall can nearly always be regarded as a modification for the 

 purpose of offering less resistance to osmosis. This is illustrated 

 by the air vesicles in the bodies of insects, which are generally 

 considered to be reservoirs for storing air to be used during 

 extended muscular exertion. The presence of these immense 

 vesicles in the metatarsi cannot be explained on the same prin- 

 ciples, for it is impossible that they should serve as reservoirs 

 for air to be used in respiration, on account of their distance 

 from the body of the insect. It is more probable that they 

 may bear some relation to the insect's method of locomotion. 

 When flying, Bittacomorpha uses the wings scarcely at all, 

 relying in great measure upon wind currents for transportation. 

 The legs are exceedingly light, as the exoskeleton is thin and 

 delicate, and encloses practically no tissue which can serve to 

 increase their weight. As they expose a large surface, they 

 offer great resistance to the air without adding appreciably to 

 the insect's weight. 



Drifting along thus, their extremely slender bodies and white 

 banded appendages give them a most peculiar, intangible ap- 

 pearance, which is heightened by their extremely slow motion. 



When examined in the cabinet, the conspicuous white and 

 black banding of Bittacomorpha seems to point toward a case 

 of warning coloration. When they are seen against their 

 natural background, however, all these brilliant contrasts fade 

 away into a perfectly neutral color which causes them to resem- 



