510 



MOTIONS OF INSECTS. 



arc of which is alternately to right and left. The rapidity of their flight 

 is so oreat that, could it be calculated, it would be found, the size of the 

 creature considered, far to exceed that of any bird, as has been proved by 

 the observations of a traveler in a railway carriage proceeding at the rate 

 of twenty miles an hour, which was accompanied, though the wind was 

 against them, for a considerable distance by a humble-bee (Bomhus subin- 

 terruptus K.), not merely with the same rapidity, but even greater, as it 

 not unfrequently flew to and fro about the carriage or described zigzag 

 lines in its flight.^ The aerial movements of the hive-bee are more direct 

 and leisurely. When leaving the hive for an excursion, I have observed 

 that as soon as they come out they turn about as if to survey the entrance, 

 and then, wheeling round in a circle, fly off. When they return to the 

 hive, they often fly from side to side, as if to examine before they alight. 

 When swarming, the heads of all are turned towards the group at the 

 mouth of their dwelling ; and upon rising into the air these little creatures 

 fly so thick in every direction, as to appear like a kind of net-work with 

 meshes of every angle. The queen 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.^ The object of these gyrations is probably to increase her chance 

 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 {Pom- 

 pilus 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 Diptera, consists altogether of two-winged flies ; 

 but, to replace the under wings of the tetrapterous insects, they are fur- 

 nished with poisers, and numbers of them also with winglets. The poisers 

 (halteres) are little membranaceous threads placed one under the origin of 

 each wing, near a spiracle, and terminated by an oval, round, or triangular 

 button, which seems capable of dilatation and contraction. The animal 

 moves these organs with great vivacity, often when at rest, and probably 

 when flying. Their winglets (alulce) are different from those of Dytiscus 

 marginalis, and the moth before noticed. 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. Derharn thinks they are for keeping the body steady in flight ; and 

 asserts that if either a poiser or winglet be cut oft', the insect will fly as if 

 one side overbalanced the other, till it falls to the ground ; and that if both 

 be cut off", they will fly awkwardly and unsteadily, as if they had lost some 

 very necessary part.^ Shelver cut off" the winglets of a fly, leaving both 

 wings and poisers, but it could no longer fly. He next cut off" the poisers 

 of another, leaving the wings and winglets, and the same result followed. 

 He found, upon removing one of these organs, that they were not properly 

 compared to balancers. Observing that a common crane-fly (Tipula 



' Philos. Mag., quoted in Burmeister's Manual of Ent. 464. ' Huber, i. 38. 



» Fhys. Theol. 13th ed. 36(). note (».) 



