4 8 
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 rr x id 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 
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 antennge 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 flagellum, although sometimes reduced 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 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 
