A pair of antennæ spring from the front of the head. In the male of the common Cockroach they are a little longer than the body; in the female rather shorter. From seventy-five to ninety joints are usually found, and the three basal joints are larger than the rest. Up to about the thirtieth, the joints are about twice as wide as long; from this point they become more elongate. The joints are connected by flexible membranes, and provided with stiff, forward-directed bristles. The ordinary position of the antennæ is forwards and outwards.
Each antenna is attached to a relatively large socket (fig. [15]), which lies between the epicranium and clypeus, to the front and inner side of the compound eyes. A flexible membrane unites the antenna to the margin of the socket, from the lower part of which a chitinous pin projects upwards and supports the basal joint.
It is well known that in many Crustacea two pairs of antennæ are developed, the foremost pair (antennules) bearing two complete filaments. Some writers have suggested that both pairs may be present in Insects, though not simultaneously, the Crustacean antennule being found in the larva, and the Crustacean antenna in the adult. This view was supported by the familiar fact that in many larvæ the antennæ are placed further forward than in the adult. The three large joints at the base of Orthopterous antennæ have been taken to correspond with those of Crustacean antennules, and it has been inferred that in Insects with incomplete metamorphosis, only antennules or larval antennæ are developed.[45] This reasoning was never very cogent, and it has been impaired by further inquiry. Weismann has shown that in Corethra plumicornis, the adult antenna, though inserted much further back than that of the larva, is developed within it,[46] and Graber has described a still more striking case of the same thing in a White Butterfly.[47] There is, therefore, no reason to suppose that Insects possess more than one pair of antennæ, which is probably preoral, not corresponding with either of the Crustacean pairs.
We have already noticed (p. 26) the superficial points in which the antenna of the male Cockroach differs from that of the female.
The eyes of some Crustacea are carried upon jointed appendages, but this is never the case in Insects, though the eye-bearing surface may project from the head, as in Diopsis and Stylops. Professor Huxley[48] supposes that the head of an Insect may contain six somites, the eyes representing one pair of appendages. The various positions in which the eyes of Arthropoda may be developed weakens the argument drawn from the stalk-eyed Crustacea. Claus and Fritz Müller go so far on the other side as to deny the existence of an eye-segment even in Crustacea.
Mouth-parts of the Cockroach.
Before entering upon a full description of the mouth-parts of the Cockroach, which present some technical difficulties, the beginner in Insect anatomy will find it useful to get a few points of nomenclature fixed in his memory. Unfortunately, the terms employed by entomologists are at times neither convenient nor philosophical.
There are three pairs of jaws, disposed behind the labrum, as in the diagram:—
Labrum.