In the Cockroach the mandibles and maxillæ are the only important instruments of mastication. The labium is indirectly concerned as completing the mouth behind and supporting the lingua, which is possibly of importance in the ordinary operations of feeding. Plateau[53] has carefully described the mode of mastication as observed in a Carabus, and his account seems to hold good of biting Insects in general. The mandibles and maxillæ act, as he tells us, alternately, one set closing as the others part. The maxillæ actually push the morsel into the buccal cavity. When the mandibles separate, the head is slightly advanced, so that the whole action has some superficial resemblance to that of a grazing quadruped.
The palps of the maxillæ and labium have been variously regarded as sensory and masticatory instruments. Not a few authors believe that they are useful in both ways. The question has lately been investigated experimentally by Plateau,[54] who finds that removal of both maxillary and labial palps does not interfere either with mastication or the choice of food. He observes that in the various Coleoptera and Orthoptera submitted to experiment the palps are passive while food is being passed into the mouth.
Plateau’s experiments are conclusive as to the subordinate value of the palps in feeding. The observation of live Cockroaches has satisfied us that the palps are constantly used when the Insect is active, whether feeding or not, to explore the surface upon which it moves. We have seen no ground for attributing to the palps special powers of perceiving odours or flavours, nor have we observed that they aid directly in filling the mouth with food.
It is worthy of note that Leydig has described and figured in the larva of Hydroporus (?), and Hauser in Dytiscus, Carabus, &c., a peculiar organ, apparently sensory, which is lodged in the maxillary and labial palps. It consists of whitish spots, sometimes visible to the naked eye, characterised by unusual thinness of the chitinous cuticle and by the aggregation beneath it of a crowd of extremely minute sensory rods. Of this organ no satisfactory explanation has yet been given.[55]
Comparison of Mouth-parts in different Insects.
The jaws of the Cockroach form an excellent standard of comparison for those of other Insects, and we shall attempt to illustrate the chief variations by referring them to this type.[56] Mouth-parts are so extensively used in the classification of Insects that every entomologist ought to have a rational as well as a technical knowledge of their comparative structure. No part of Insect anatomy affords more striking examples of adaptive modification. In form, size, and mode of application the jaws vary extremely. It would be hard to find feeding-organs more unlike, at first sight, than the stylets of a Gnat and the proboscis of a Moth, yet the study of a few well-selected types will satisfy the observer that both are capable of derivation from a common plan. Nor is this common plan at all vague. It is accurately pictured in the jaws of the Cockroach and other Orthoptera. These correspond so entirely with the primitive arrangement, inferred by a process of abstraction from the most dissimilar Insects, as to furnish a strong argument for the descent of all higher Insects from forms not unlike Orthoptera in the structure of their mouth-parts.
Fig. 21.—Embryo of Aphis. Copied from Mecznikow, Zeits. f. wiss. Zool., Bd. XVI., taf. xxx., fig. 30. References in text. × 220.
Though the jaws of the Cockroach are eminently primitive with respect to those of most other Insects, they are themselves derived from a far simpler arrangement, which is demonstrable in all embryonic Insects. Fig. 21 shows an Aphis within the egg. The rudiments of the antennæ (At), mandibles (Mn), and maxillæ (Mx1, Mx2) form simple blunt projections, similar to each other and to the future thoracic legs (L1, L2, L3). We see, therefore, that all the appendages of an Insect are similar in an early stage of growth; and we may add that a Centipede, a Scorpion, or a Spider would present very nearly the same appearance in the same stage. A Crustacean in the egg would not resemble an Insect or its own parent so closely.[57] Aquatic life favours metamorphosis, and most Crustacea do not begin life with their full quota of legs, but acquire them as they are wanted.