THE INSECT WHOSE VOICE IS IN ITS LEGS

YOU have often heard grasshoppers on the commons; they are not calling to you but to each other. The grasshopper’s note is obtained by drawing its hind-legs across the edges of the fore-wings; the inner edge of the leg bears a row of pegs which are rubbed against the wing to produce a high-pitched note by friction. The frictional method by which one part of the body is rubbed against another part is by far the commonest method employed in calling. The vibration of wings in the bumble bee produces sound. The Death watch beetle in old furniture taps its head on the wooden floor of its tunnel and produces a noise like, the ticking of a watch.

Sound may be used as an attracting or warning signal in insects. Insects calling to each other implies some sort of hearing apparatus in the group. The grasshopper’s ears are

in its abdomen ! These ears are complicated structures but they are easily recognised. An oval membrane is found at the sides of the first abdominal ring; sound waves cause it to vibrate, and these vibrations are passed on to the appropriate nerve. There are other methods of sounding and hearing, but these examples will serve to show that ways exist.

A FEARSOME ARMOURY FOR BITING AND SUCKING THERE are two methods of feeding employed by insects— biting and sucking. Primitive insects show the typical biting condition, and a study of their mouth-parts reveals three pairs of jaws : the strong, tooth-like mandibles and two other pairs called maxilla? or accessory jaws. They are well seen in the cockroach. A biting insect is able to feed on solid particles of food which are torn up by the mandibles

and shovelled into the mouth by the accessory j’aws. In higher insects which feed on nectar, the mouth-parts have become adapted to sucking. We may cite the butterflies as examples of insects with mouth-parts considerably modified to meet these requirements. Other insects have mouth-parts with both biting and sucking constituents and are therefore intermediate between these two extremes.

The mandibles have a crushing surface in insects which feed on plants; in carnivorous insects they are pointed. Such crushing and pointed mandibles are found in grasshoppers, sawflies and beetles. In many male insects, as, for instance, in the Stag beetle, the mandibles are very much larger than those of the female and are used as weapons in addition to their usual function. The wasp has mandibles for chewing wood to pulp, and in general form these are similar to those of the cockroach; the bee, on the other hand, has club-shaped mandibles used in comb-building. Similar modifications in connection with feeding occur in connection with the other two pairs of jaws and in the more highly specialised insects the two tend to fuse.

In general, the mouths and jaws of most mature insects are more highly organised than those of the primitive biting type of the cockroach and larvae. In butterflies, the second pair of jaws (maxilla;) are prolonged into a sensitive sucking tube for reaching down to the nectaries of tubular flowers. In bees and flies, the third pair of jaws (second maxilla;) are similarly modified. The mouth of the female mosquito shows us quite a set of implements for piercing the skin and sucking the blood. The mandibles and maxilla; are prolonged into fine blades; the maxilla; are furnished with little saws at their tips, and there is also an unpaired lancet and sucking tube for drawing up the blood. The male mosquito, which does not suck blood, is devoid of these specialisations.

All these modifications have interesting biological reactions : an insect whose mouth-parts have undergone little specialisation can feed on widely different things, and hence its distribution is not seriously hampered by the scarcity of one particular kind of food. As specialisation increases, the insect’s distribution depends upon food factors and its range becomes limited by these, until we find certain insects confined to localities where a particular plant grows; blood-sucking flies, like the tsetse fly, are confined to regions inhabited by the animals upon which they feed.

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