354 MOTIONS OF INSECTS. 



also, upon going forth, when her object is to pair, after 

 returning to reconnoitre, begins her flight by describing 

 circles of considerable diameter, thus rising spirally with 

 a rapid motion a . The object of these gyrations is pro- 

 bably to increase her enhance of meeting with a drone. 

 I have not much to tell you with respect to the flight of 

 other insects of this order, except that a spider -wasp 

 (Pompilus viaticus) whose sting is redoubtable, and which 

 often, when we are in the vicinity of sandy sunny banks, 

 accompanies our steps, has a kind of jumping movement 

 when it flies. 



The next order, the Diptcra, consists altogether of 

 two-winged flies: but to replace the under wings of 

 the tetrapterous insects, they are furnished with poisers, 

 and numbers of them also with winglets. The poisers 

 (Halter es) are little membranaceous threads placed one 

 under the origin of each wing, near a spiracle, and ter- 

 minated by an oval, round, or triangular button, which 

 seems capable of dilatation and contraction. The ani- 

 mal moves these organs with great vivacity, often when 

 at rest, and probably when flying. Their winglets 

 (Alulae) are different from those of Dytiscus marginalis, 

 and the moth before notjced. Like them, they are of 

 rigid membrane, and fringed; but they consist generally 

 of two concavo-convex pieces (sometimes surrounded by 

 a nervure), situated between the wing and the poisers, 

 which, when the insect reposes, fold over each other 

 like the valves of a bivalve shell ; but when it flies they 

 are extended. The use of neither of these organs seems 

 to have been satisfactorily ascertained. Dr. Derham 

 thinks they are for keeping the body steady in flight; 

 a Huber, i. 38. 



