MOTIONS OF INSECTS. 475 



generally bodies thicker than those of most other Hymenoptera, while those 

 that have fewer nervures are more slender. This, however, does not 

 hold good in all cases so that the dimensions and cut of the wings, the 

 strength of their nervures, and the force of their muscles, must also be 

 taken into consideration. The wings of many of these insects, when 

 expanded, are kept in the same plane by means of small hooks (hamuli) 

 in the anterior margin of the under wing, which lay hold of the posterior 

 margin of the upper. 1 Another peculiarity also distinguishes them. Base 

 covers (teguke), or small concavo-convex shields, protect the base of the 

 wings from injury 2 or displacement. 



The most powerful fliers in this order are the humble-bees, which, like 

 the dung-chafers (Geotropes), traverse the air in segments of a circle, the 

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

 so great that, could it be calculated, it would be found, the size of the crea- 

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

 observations of a traveller 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 (Bombus subinterruptus 

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

 quently flew to and fro about the carriage or described zig-zag lines in its 

 flight. 3 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 consider- 

 able diameter, thus rising spirally with a rapid motion. 4 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 (Pompilus viaticus), whose sting is redoubt- 

 able, 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 furnished 

 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 (alula;) are different from those of Dytiscus mar" 

 ginalis, 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 



1 Kirby, Mon. Ap. Angl. i. 96. 108. t. xiii. f. 19. 



2 Ibid. i. 96. 107. t. v. f. 8. dd. 



5 Philos. Mag., quoted in Burmeister's Manual of Ent. 464. 

 4 Huber, i. 38. 



