48 



INSECTS. 



are almost all longitudinal, that is, they run from the base or point of attachment 

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

 figures by the letters a, b, c,-d, e,f, 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 1, 2, 3, etc.) circumscribed by these veins form valuable systematic 

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

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

 concealed by a scale-like membrane as in the blue-bottle fly. In connection with 

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

 two distinct 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 blow-fly 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 

 humble-bee 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 possess the normal five segments ; the tarsi or feet, which are 



WING OF DADDY-LONGLEGS AND OF BLOW-FLY. 



also divided into five segments, 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 considerably 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 extre- 

 mity of the terminal part of the organ, which frequently has the form of a plume- 

 like whip, the flagellurn, although sometimes reduced to a bristle. Not unf requently 

 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 females of most of the common flies, on the 

 contrary, may be recognised 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 between them. Rarely the sexual characters are 



