Again: when, in his first mode of articulation by contact of solid parts, he speaks of one or two smooth tubercles of the neck, with their corresponding cavities in the prothorax, as forming the most common conformation, you would expect to find examples of this in very many insects; yet upon a close examination, unless in Oryctes nasicornis[1178], and perhaps in others of the Dynastidæ MacLeay, you would scarcely meet with any thing that could be called a tubercle and its corresponding cavity in the neck or prothorax of any Lamellicorn or Capricorn beetle that you might chance to examine. You would find, indeed, that the occiput was usually smooth and very slippery, as if lubricated; that in its margin were one or two notches (Myoglyphides), with muscles attached to them; that in the former of these tribes, the Lamellicorns, it projected on each side so as to form a more or less prominent angle; and that the throat (jugulum) was very convex, and lodged in a cavity of the lower margin of the prothorax: but further appearances of tubercles &c. you would in vain look for even in this tribe; and in the Capricorns you would find that the general conformation in this respect belonged to our learned author's second mode of solid articulation, resembling that of the majority of the weevils (Curculio L.), in which the head has no projecting angles or tubercles, or other elevation, but is received usually into the circular cavity of the prothorax.
His third mode of this articulation, that of the Hymenoptera and Diptera, is so peculiar, that it ought to be considered as a primary kind; since in this the head moves upon the prothorax as upon a pivot, and has a kind of versatile motion.
With regard to his fourth mode, which from his description appears to be that of Apoderus Oliv., he allows motion to the head only in one direction, observing that the lower margin of the prothoracic cavity has a notch. But M. Latreille calls the articulation of the head in this genus an Enarthrosis[1179], which admits motion in every direction; and if you examine the common species (A. Coryli), you will find that the prothorax has a sinus taken out of its upper margin, as well as out of its lower one—which at any rate will allow a motion upwards.
I merely mention these little inaccuracies, with due diffidence, as some apology for giving you a different and at least a more popular and general view of this part of my subject, which I shall now proceed to state to you. It seems to me most convenient for the Entomologist, and most consonant to nature, to divide insects, with respect to the articulation of the head with the trunk, into three primary sections, each admitting one or more subdivisions.
1. The first consists of those whose head inosculates more or less in the anterior cavity of the chest; and whose articulation, therefore, seems to partake in a greater or less degree of the ball and socket (Enarthrosis). The head, however, is often capable of being protruded from this cavity. If you take into your hand any common Harpalus that you may find under a stone, you will see, if pressed, that it can shoot forth its head, so as to be entirely disengaged from the prothorax: a neck of ligament intervening between them[1180]: of course this power of protruding the head enables the animal to disengage it at its will from the restriction imposed upon its motions by the surrounding margin of the prothoracic cavity. To this section belong all the Coleoptera, the Heteropterous Hemiptera (Cimex L., &c.), and some of the Neuroptera (Raphidia, Semblis, &c.).—It may be further divided into two subsections—those, namely, whose head inosculates in the prothorax by means of a neck: as for instance Latreille's Trachelides, Apoderus, and the Staphylinidæ, amongst the beetles; the Reduviadæ amongst the Heteropterous insects, and Raphidia in the Neuroptera; and those whose head inosculates in the prothorax without the intervention of a neck; as, the Petalocera, the aquatic beetles (Dytiscus, Hydrophilus, &c.), and most of the genus Curculio L. in the first of these orders, the great body of the Cimicidæ in the second, and Semblis, Corydalis, &c. in the third.
2. The second section consists of those insects whose head does not inosculate in the chest, but is merely suspended to it by ligament or membrane. To this belong most of the tribes of the Orthoptera Order, with the exception of the Mantidæ, the Dermaptera, the Homopterous Hemiptera, and such of the Aptera as have the head distinct from the prothorax.—This section admits of a triple subdivision: those, namely, whose head is wholly covered by the shield of the prothorax, as in Blatta L.; those whose head is partly covered by it, as Gryllotalpa, and other Gryllina; and those whose head is quite free, not being at all impeded in its motion by the prothorax, as the Dermaptera, Nirmus, Pediculus, &c.
3. The third section consists of those whose head is truncated posteriorly, and flat or concave, with a very small occipital aperture, and is attached to a neck of the prothorax upon which it turns, or is merely suspended to that part. This includes the Lepidoptera, Hymenoptera, Diptera, the Libellulina, &c. in the Neuroptera, and the Mantidæ in the Orthoptera. Three subsections at least, if not more, present themselves in this section: the first is, of those whose head is united to the prothorax, without the latter forming any neck. To this belong the Lepidoptera, Trichoptera? The second is of those the upper side of whose thoracic neck is ligamentous; and here you may range most of the Hymenoptera. The third is of those in whom it is a continuation of the ordinary integument. In this subsection the Diptera, Libellulina and Mantidæ will find their place. In this last section the head appears to turn upon the thorax as upon a pivot.
Before I finish what I have to say on the articulation of the head, I must direct your attention to the analogies that hold in this respect between the different tribes. Thus the Coleoptera are analogous to the Heteropterous Hemiptera; the Orthoptera, with the exception of the Mantidæ, to the Homopterous Hemiptera; the Mantidæ to the Libellulina; the Lepidoptera to the Trichoptera; the Hymenoptera to the Diptera, with a slight variation, and probably others might be traced.
viii. A word or two upon the motions of which the head of insects is capable. M. Cuvier, in the extracts lately laid before you, speaks of different powers of movement as the result of different configurations of the parts of the head. This probably is correct with regard to many cases; but a great deal will depend upon the power the insect has of protruding its head and disengaging its base from the restriction of the prothorax; for where, like the Harpali, Staphylini, &c. it is able to do this, it can probably move its head in every direction. It is only where the ligaments are less elastic, or allow of little tension, that its movements are confined; and few living insects have been sufficiently examined to ascertain how far this takes place. In those cases belonging to the third section of articulations, in which the head moves upon the thorax as upon a pivot, as is the case with Hymenoptera and Diptera, the movement is nearly versatile. I have seen a fly turn its head completely round, so that the mouth became supine and the vertex prone; and from the form and fixing of the head, it should seem that those of the Mantidæ were endued with the same faculty.