edge is indented like a bean, or like the kidneys of vertebrates. The eyes are then said to be reniform.
The organ of vision is sometimes made more complete by simple eyes, detached from the general group and permitting the insect to distinguish parts of space outside the field of the faceted
Fig. 11.—Section of an Insect's Eye. C, cornea; fac, cones; hr, rods; P, pigmentary sheathings of the rods; go, ganglion of the optic nerve; no, optic nerve. (According to Nuhn.) eyes. These supplementary organs, called ocelli, usually three in number, are arranged triangularly between the reticulated eyes.
The antennæ are articulated appendages which insects bear on the head, near the eyes, sometimes forward of them, sometimes behind them, and which are often greatly developed.
The antennæ are generally regarded as organs of touch; there is hardly any doubt about the fact. Some naturalists make them the seat of the smell; others consider them connected with hearing; and some locate both senses in them. However it may be, they are formed of a series of articulations united to one another, the number and form of which furnish entomologists with good characteristics for classification.
The antennae are straight or bent. The basilar joint, which is in direct relation with the head, is called the scape; in bent antennae it is usually very large, and forms an obtuse angle with the next joint. The club, or terminal part of the antenna, is sometimes in the shape of an olive, and is composed of a variable number of joints. The whole number of joints between the scape and the club constitutes the funicle. Characteristic forms of antennæ are represented in Fig. 12.
The thorax comprises three rings more or less closely joined: the prothorax, mesothorax, and metathorax. The prothorax bears the first pair of legs; it is largely developed in the Coleoptera and Hemiptera, in which it appears as a horny buckler, and is freely articulated with the mesothorax. It is the corselet of the old authors, and is called the pronotum by the entomologists of to-day.
The mesothorax bears the second pair of legs and the first pair of wings, which are sometimes horny; we perceive on the upper part of the insect only a small triangular portion of it, which is hardly visible in some species, and is called the scutellum or escutcheon. All the rest of the dorsal part is covered by the wings.
The metathorax is closely united to the preceding ring, and frequently also to the first rings of the abdomen. It bears the
