MOTIONS OF INSECTS. 



283 



The four wings of the next order, the Trichoptera or case- 

 worm flies, both in their shape and nervures resemble those 

 of many moths 1 ; only instead of scales they are usually co- 

 vered with hairs, and the under wings, which are larger than 

 the upper, fold longitudinally. Some of these flies, I have 

 observed, move in a direct line, with their legs set out, which 

 makes them look as if they were walking in the air. In flying 

 they often apply their antennas to each other, stretching 

 them out straight, and thus probably are assisted in their 

 motion. 



The Lepidoptera vary so infinitely in the shape, comparative 

 magnitude, and appendages of their wings, that I should detain 

 you too long did I enlarge upon so multifarious a subject. I 

 shall therefore only observe, that one species is described, 

 both by Lyonet and De Geer 2 (Lohophora hexapterd), as 

 having six wings ; for, besides the four ordinary ones, it has a 

 winglet (alula) attached to the base of the lower one, and 

 placed, when the wings are folded, between it and the upper. 

 These organs in this order, you know, are covered with scales 

 of various shapes. Their nervures are diverging rays, which 

 issue either from a basal area or from the base itself, and 

 terminate in the exterior margin. The wings of many male 

 butterflies, hawk-moths, and moths, are distinguished by a 

 remarkable apparatus, noticed by De Geer, and since by many 

 other naturalists 3 , for keeping them steady and underanged 

 in their flight. The upper wings, on their under side near 

 their base, have a minute process, bent into a hook (hamus), 

 and covered with hairs and scales. In this hook one or more 

 bristles (tendo), attached to the base of the under wing, have 

 their play. When the fly unfolds its wings, the hook does 

 not quit its hold of the bristle, which moves to and fro in it 

 as they expand or close. The females, which seldom fly far, 

 often have the bristles, but never the hook. The hairy tails 

 of some insects (Sesia) belonging to the hawk-moth tribe are 

 expanded when they fly, so as to form a kind of rudder, which 

 enables them to steer their course with more certainty. 



1 Plate III. Fig. 4. 



2 Lesser, 1. i. 10.9, note * De Geer, ii. 460. t. ix. f. 9. 



3 De Geer, i. 173. t. x. f. 4. Linn. Trans, i. 135. 



