MOTION. 



423 



to the cubic contents of the body, endows this 

 insect with great powers of suspension in the 

 air and with great velocity of motion. The 

 triangular figure of the wings prevents the un- 

 steady undulating progression observed in the 

 diurnal Lepidoptera, and the flight is conse- 

 quently more direct as well as more rapid. The 

 wings of many of the moths are of considerable 

 dimensions. The largest male Atlas moth in 

 the collection at the British Museum measures 

 5f inches on each side, estimated from their 

 axes of motion to the apices of the wings, pre- 

 senting a total area of 26% square inches. If 

 the force of the muscles acting on the wings is 

 proportional to their areas, they must possess 

 the most extensive power of flight. The Bombyx 

 mori, or silk-worm moth, is stated to travel 

 more than a hundred miles a day.* 



The Neuroptera have a separate set of 

 muscles appropriated to the movements of 

 each wing, which being detached can be 

 moved either synchronously or independently 

 of each other : the muscles also are actually 

 inserted into the wing, instead of moving 

 them indirectly, as in other orders of insects. 

 The surfaces of the four wings of the Libellulce, 

 or dragon-flies, are nearly equal in most species ; 

 they are always expanded in repose, and ex- 

 tended horizontally at right angles to the axis 

 of the body, so that they take flight in an in- 

 stant. Their figure is lanceolated. The ratio 

 of the united areas of the four wings to the 

 weight of the body is greater than in the Cole- 

 optera and Hymenoptera, and the muscular 

 power is proportionally augmented. The arti- 

 culation of the wings being situated above the 

 centre of gravity keeps them steady in flight : 

 their velocity is very great, exceeding that of 

 the Swallow. Leeuwenhoek once observed one 

 of this tribe in a menagerie 200 feet long, 

 chased by a swallow ; the insect flew with such 

 velocity, and turned to the right and left in all 

 directions so instantaneously, that the swallow, 

 with all its powers of flight and tact in chasing- 

 insects, was unable to capture it, the insect 

 always keeping about six feet in advance of the 

 bird. The great length and surface of the 

 wings of the Libellulse, and the power of the 

 muscles acting on them, is such that they appear 

 to be never tired of flying in quest of their prey. 



In a specimen of the TEshna maculatissima, 

 which weighed fourteen grains, the area of the 

 anterior wing was 0.7324 in. the posterior 

 0.8988 in. and the area of the four wings 

 3.26408 square inches. The preponderance of 

 surface in the posterior wing enables the Libel- 

 lulse to change the direction of their path of 

 motion with great facility, and to capture their 

 prey on the wing. Previously to taking flight, 

 this jshna exerts a vibratory movement with 

 its wings; the oscillations are made in very 

 minute arcs, and with great rapidity, producing 

 a faint though distinct sound. The pitch indi- 

 catesthat the wings perform ninety-six vibrations 

 in a second. On taking flight the wings oscil- 

 late through larger arcs, with a less number of 

 vibrations, the amount of which it is not easy 



* Lin. Trans, vol. iii. p. 40. 



to determine, and does not depend, as some 

 distinguished entomologists have supposed, on 

 their mutual friction. 



In a Triphana pronuba, weighing 8.545 

 grains, the area of the anterior wings measured 

 0.6213, and that of the under wings 0.68 

 square inches, making the sum of the areas of 

 the four wings 2.6026 square inches. 



In the Hymenoptera the ratio of the areas 

 of the four wings to the weight of the 

 insect is less than in the Neuroptera, and 

 they are consequently obliged to make a 

 greater number of strokes in the same inter- 

 val of time in order to suspend themselves 

 in the air. The areas of the upper are greater 

 than those of the under wings. When ex- 

 panded they are retained in the same place, 

 and are linked together by means of small 

 hooks, so that the upper and under wings act 

 simultaneously and with greater power. A 

 humble bee, which weighed 6.2 grains, had 

 wings the sum of whose areas measured 0.366 

 of a square inch, or rather more than J 5 th of a 

 square inch to each grain weight of the body. 

 Bees are not only celebrated for the geo- 

 metry displayed in the structure of their cells, 

 but also for the precision with which they re- 

 turn to their homes by the shortest path or in a 

 straight line. The collectors of honey make 

 use of this fact to discover their nests. Having 

 captured two of these insects, they separate 

 them some yards from each other, and on setting 

 them free, ascertain with an instrument the 

 angles respectively made by the lines of their 

 flight with that between the points of their de- 

 parture, then the point where the two lines of 

 direction intersect each other indicates the posi- 

 tion of the nest. The humble bee, wasp, and 

 hornet fly with great force, but owing to the 

 weight of their bodies, compared with the areas 

 of their wings, cannot fly with much speed 

 against a strong wind, so that a person might 

 easily outstrip them by running in the same 

 direction against the wind. When disturbed, or 

 before leaving their abode, they wheel round the 

 spot in a large circle, and then fly off at a tangent 

 to some part of the curve. The Ichneumons are 

 provided with a far greater expansion of wing, 

 in proportion to their weight, than the Bees, 

 and can consequently sustain themselves in the 

 air with less expenditure of muscular action. 

 In a species of ichneumon allied to Ophion 

 luteus, which weighed 0.5 gr. the areas of the 

 anterior wing measured 0.0832 in. the poste- 

 rior 0.0480 in. the sum of the two wings = 

 0.1322 in. and that of the four = 0.2644 in. 



The Diptera are furnished with only two 

 wings, which when in repose lie directed 

 obliquely backwards upon the abdomen; their 

 figure is nearly that of an ellipse, and their 

 areas are ample when compared to the weight 

 of their bodies. Three examples of Muscte vo- 

 mltor'uc for instance were found to weigh 2.4375 

 grains, which gives for the mean weight of each 

 0.8025 gr. and the mean areas of both wings 

 were found to be from -^th to ^th of a square 

 inch. Instead of posterior wings they have 

 poisers, the articulation of which to the thorax 

 is placed more posteriorly than in the four- 



