MOTIONS OF INSECTS. 507 



varying from membrane to a leathery or horny crust^ — and those in which 

 the base and the apex are of different substances ; the first being generally 

 corneous, and the latter membranaceous.^ The former or homopterous 

 division includes the Cicadance, Latr., Aphis, Chermes, Thrips, and Coc- 

 cus ; — and the latter the heteropterous division, comprehending, besides 

 the Geocoriste Latr., Notenecta, Sigara, Nepa, Ranatra, and Naucoris of 

 Fabricius. The posterior tibiae of some of this last division {Lygceus 

 phi/Uojms,foUac€us, &c. F.) are furnished on each side with a foliaceous 

 process — which may act the part of outriggers, and assist them in their 

 flight.^ I can give you no particular information with respect to the aerial 

 movements of the insects of this order: the British species that belong to 

 it are generally so minute that it is not easy to trace them with the naked 

 eye ; and unless some kind optician, which is much to be wished, would 

 invent a telescope by which the proceedings of insects could be examined 

 at a distance, there is no other way of studying them. 



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

 both in their shape and nervures resemble those of many mollis^ ; only 

 instead of scales they are usually covered 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 antennae 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^ {Lohophora 

 hexaptera), us 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 remarka- 

 ble apparatus, noticed by De Geer, and since by many other naturalists^, 

 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. 



The insects of this and of every other order, except the Coleoptera, 



1 Plate II. Fig. 4. « Plate II. Fig. 5. 



3 I have separated this tribe from the rest under the name of Petalopu, K. Ms. 



* Plate III. Fig. 4. 



s Lesser, 1. i. 109, note*. De Geer, ii. 460. t. ix. f. 9. 



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



