The head capsule is more or less freely movable on the thorax, and bears certain sensory organs, together with the mouth appendages. The sensory organs are the eyes and the feelers, or antennæ. On each side is a compound eye of varying size, according to the insect; each eye consists of a variable number (from a comparative few to several thousand) of microscopic, hexagonal lenses, each of which records a separate image. Between the compound eyes, on top of the head, are three simple eyes in some insects, but in others one or all of these may be absent. Between the compound eyes on the front aspect of the head is a pair of feelers, or antennæ; they consist of a variable number of joints, are freely movable and highly sensory, thread-like or hair-like, short, or longer even than the whole body, and may be bare or clothed to a varying degree with hair or bristles. On the antennæ are the organs of touch, smell, and sometimes hearing.
FIG. 3.
When the head of a weta, grasshopper, or cockroach is removed from the body and boiled for a few minutes in a 10 per cent. solution of caustic potash, and then washed in water in order to remove the muscles and other tissues, a large opening will be seen on the posterior surface where the head was attached to the thorax; also, if the mouth appendages are pulled apart, they will be seen to surround another opening on the lower aspect of the head capsule, marking the position of the mouth. The digestive canal passes from the mouth through the posterior opening into the thorax.
The mouth appendages are as follows ([Fig. 3]):—Suspended from the fore aspect of the mouth opening is a more or less conspicuous movable flap, which forms the upper lip, while from the posterior aspect of the same opening is another suspended appendage forming the lower lip; this latter appendage is really a complicated one, and bears a pair of short, jointed appendages—the palps—which are sensory organs, while on its inner surface—i.e., within the mouth—is a swollen area or tongue, an organ very greatly modified in certain insects. Between the upper and lower lips, and suspended from both sides of the mouth opening, is a pair of true jaws immediately behind the upper lip, followed by a pair of accessory jaws immediately before the lower lip; these jaws do not move up and down, but have a side-wise action, closing and opening like scissor blades. While the true jaws are each of one piece, the accessory jaws consist of several parts, and each bears in addition a jointed palp, as in the case of the lower lip. The upper and lower lips serve to hold the food in the mouth, the true jaws nibble or tear off portions of the food and masticate it (if the term can be used), while the accessory jaws, aided by the lower lip, manipulate the food during the process of feeding.
The comparatively simple arrangement of mouth parts found in the weta, grasshopper, and cockroach, as described above, is characteristic of all insects that gnaw or chew their food—e.g., earwigs, beetles and their larvæ or grubs, the caterpillars of moths, and so on. There is, however, a vast number of insects that has developed more or less complex variations of this generalised pattern, according to the manner of feeding.
The mouth parts of the worker honey-bee, for example, have the jaws adapted for eating pollen and moulding wax for the comb; the accessory jaws, however, are lengthened, though their palps are reduced to mere vestiges in contrast with the elongated palps of the lower lip; the most remarkable modification is that of the greatly elongated tongue, with its spoon-like tip adapted for reaching nectar of flowers having deep-seated nectaries. For the same purpose, the mouth parts are modified in a moth ([Fig. 3]) to form a long proboscis, which lies curled up in a spiral beneath the head when not in use; in this case the proboscis is the modified accessory jaws, the remaining mouth parts, with the exception of the well-developed palps of the lower lip, being greatly reduced. In a blood-sucking insect, such as the female mosquito, all the mouth parts are well developed, but are very delicate and greatly lengthened and suited for piercing the skin. The greatest modification is found in the blow-fly proboscis, which is a soft, sucking tube, with no outward resemblance to the generalised plan, except for the palps of the accessory jaws. The mouth parts of insects (e.g., aphids) which feed on the nutrient sap of plants, just in the same way as mosquitoes do on blood, are modified for puncturing the tissues of plants; in such insects the upper lip is short, and both pairs of palps are atrophied, but the jaws and accessory jaws are greatly lengthened in the form of bristle-like stylets, which lie in a groove along the equally lengthened lower lip ([Fig. 3]). The manner in which insects feed is of great importance in controlling them with insecticides, and the two types to bear in mind are those that chew their food and those that suck the sap of plants, reached by puncturing the tissues.
As already stated, the thorax consists of the three segments immediately behind the head, and carries the organs of locomotion; its three segments are distinct, and may be referred to, respectively, as the fore, middle, and hind thorax. The cuticle of each thoracic segment consists of a number of chitinised plates connected by membranous areas; these plates are arranged in three series—the back or dorsal; the lower or ventral, forming the sternum; and the lateral, or side-pieces, connecting the dorsal and ventral ones.