FLIES AND FLEAS 



3011 



cf the wing to its free margin. These veins are represented in the accompanying 

 figures by the letters a, b, c, d, e,f y g. The transverse veins x, y, on the contrary, 

 are always few in number. The shape and size of the spaces (indicated by the 

 numbers i, 2, 3, etc.) circumscribed by these veins form valuable systematic char- 

 acteristics for distinguishing the species and genera of this order. The balancers 

 may be entirely exposed, as in the common daddy longlegs, but are sometimes con- 

 cealed by a scale-like membrane as in the bluebottle fly. In connection with the 

 wings may be noticed the buzzing of flies. This appears to be the result of two dis- 

 tinct sounds, one produced by the rapid vibration of the wings, and the other by 

 the vibration of the thorax. The latter movement is the more rapid of the two, and 

 gives rise to the shrill note heard the moment a blowfly is seized; while the former 

 is the ordinary buzzing produced when the insect is in flight. According to recent 

 calculations, the thoracic vibrations in the case of one of the bumblebee flies 

 ( Volucella) amounted to thirteen hundred per second, while those of the wings were 

 just one-half this number, namely six hundred and fifty per second. The legs pos- 

 sess the normal five segments; the tarsi or feet, which are also divided into five seg- 



WING OF DADDY LONGLEGS AND OF BLOWFLY. 



ments, being armed with two claws, and in addition, often supplied with adhesive 

 pads, by means of which the insects are enabled to ascend perfectly smooth surfaces. 

 These pads are composed of a multitude of funnel-shaped hairs, each supposed to 

 act as a minute sucker. Some authors assert, however, that they secrete a sticky 

 fluid, and that the insect maintains its hold by this means. The antennae vary con- 

 siderably in structure. In their least modified form, as presented by the gnats and 

 their allies, they are simple and thread-like organs, consisting of a series of subequal 

 segments, often modified by the presence of long symmetrically-arranged bristles, 

 which impart to them a feather-like aspect. In most of the members of the order 

 the antennae are, however, curiously constructed. The three basal segments are 

 stout, the third being especially large and produced into a great lobe-like plate, 

 sometimes projecting as far as the extremity of the terminal part of the organ, which 

 frequently has the form of a plume-like whip, the flagellum, although sometimes re- 

 duced to a bristle. Not unfrequently the antennae differ greatly in structure- 

 according to sex. In the males of gnats, for example, they are large and feathery,, 

 while in the females they are only furnished with short hairs. The males and fe- 

 males of most of the common flies, on the contrary, may be recognized by the 

 development of the compound eyes. In the former sex these organs are almost in 

 contact on the summit of the head, while in the latter there is a widish space be- 

 tween them. Rarely the sexual characteristics are much more pronounced, as for 



