v. It is the most general rule, as to its proportion, that it shall be smaller than either trunk or abdomen; but in some instances, as in the S. American ant, Atta megacephala, it is much larger than either.
vi. By the direction of the head, I mean its inclination with respect to the prothorax. The most natural direction, or that which obtains most generally, is for it to form an angle more or less obtuse with the part just mentioned. This seems to obtain particularly in Coleoptera; but in some, as Mylabris, it is inflexed, forming an acute angle with it. In the Heteropterous Hemiptera (Cimex L. &c.) it is generally in the same line with the body, or horizontal; and in many Diptera it is vertical.
vii. We now come to a circumstance which will detain us longer, namely, its articulation with the trunk, or rather with its anterior segment, the prothorax.—M. Cuvier makes two principal kinds of articulation of the head upon the prothorax, in one of which the points of contact are solid, and the movement subordinate to the configuration of the parts; in the other, the articulation is ligamentous, the head and the thorax being united and kept together by membranes.
1. The first of these kinds of articulation—that by the contact of solid parts—takes place, he says, in four different ways. "In the most common conformation, in the part that corresponds to the neck, the head bears one or two smooth tubercles, which receive corresponding cavities of the anterior part of the prothorax observable in the Lamellicorn and Capricorn beetles. In this case the head can move backwards, and the mouth forwards and downwards. The second mode of solid articulation takes place when the posterior part of the head is rounded, and turns upon its axis in a corresponding cavity of the anterior part of the prothorax; as may be seen in Curculio, Reduvius, &c. The axis of motion is then at the centre of articulation, and the mouth of the insect moves equally backwards and forwards, upwards and downwards, to right and left.—The third sort of articulation, by solid surfaces, takes place when the head, truncated posteriorly, and presenting a flat surface, is articulated, sometimes upon a tubercle of the thorax, and sometimes upon another flat and corresponding surface, as in almost all the Hymenoptera and the majority of the Diptera. The disposition of the fourth kind of articulation allows the head only the movement of the angular hinge (le seul mouvement de charnière angulaire). The only examples at present known are in some species of Attelabus F. The head of these insects terminates behind in a round tubercle, received in a corresponding cavity of the thorax: the lower margin of this cavity has a notch, and permits the movement of the head only in one direction[1175]."
2. The second kind of articulation, the ligamentous, he affirms takes place only in orthopterous and some neuropterous insects: "The head in this kind of articulation is only impeded in its movements towards the back, because it is stopped there by the advance of the prothorax; but below it is quite free. The membranes or ligaments extend from the circuit of the occipital cavity to that of the anterior part of the prothorax, which gives a great extent to the movement[1176]."
When I consider the well-deserved celebrity of the great author whose words I have here quoted, and the great and useful light that the genius and learning which conducted his patient labours and researches have thrown over every department of comparative anatomy,—a science he may be almost said to have founded,—I feel the most intire reluctance to differ in any thing from an authority so justly venerable to all lovers of that interesting study. But, however great my diffidence and hesitation to express an opinion at all opposed to his, the interests of truth and science require that I should state those particulars in which my own observations, made upon a careful examination of various insects of every Order, have led to results in some respects different from the above. "Aliquando bonus dormitat Homerus;" and if the Genius of Comparative Anatomy ever nodded, it sometimes happened when he was examining the structure of insects. An instance of this with regard to the mouth of the bee has been noticed by Mr. Savigny[1177]; and indeed it is not wonderful that in so extensive an undertaking, in which the number of examples to be examined upon every branch of his subject must be immense, that he did not always scrutinize minutely those that seemed less important. Every writer on every department of Natural History, especially where the objects of research, as in the insect world, are so infinite in number, will be liable to such mistakes; but these will meet with due allowance from every candid mind—
"Hanc veniam damus, petimusque vicissim:"
and I shall express my trust that you will overlook any errors of mine; and doubtless I shall not be free from them—
"————-Quas aut incuria fudit,
Aut humana parum cavit natura——"
The two kinds of articulation of the head which our learned author has stated as the principal ones, will, I think, be found upon examination not so widely distant as his expressions seem to indicate; for in fact in all insects, as well as the Orthoptera, this part is suspended by a membrane or ligament which unites the margins of the occipital cavity with those of the anterior one of the prothorax: thus forming all round some protection to the organs that are transmitted from the head through the latter part to the rest of the body. Though the head in most Orthoptera is not partly received into the cavity of the prothorax, as is the case in the Order Coleoptera, but is rather suspended to it, yet in some instances, for example in the mole-cricket (Gryllotalpa vulgaris), it is partially inserted.