This mode of formation of the head may be observed still more easily in Rhodites, Hemiteles, and Microgaster, from the fact that their oculo-cephalic buds are much more precocious, and that the eyes are charged with pigment at a period when the insect still preserves its larval form.
“... I believe that this mode of formation of the head occurs in all Hymenoptera with apodous larvæ, in this sense; that a more or less considerable part of the first thoracic segment is always soldered to the head of the larva to constitute the head of the perfect insect. The arrangement of the nervous system is naturally in accord with this peculiarity of development, and the cephalic ganglia of the larva to which the ocular blastems later adapt themselves, are found not in the head, but in the succeeding segment (Figs. 39, 40, 41).
“Relying on these facts, I maintain that the encroachment of the head on the prothorax is a consequence of the preponderance in size of the brain, and indicates the superiority of the Hymenoptera over other insects....”
That the pronotum is derived from the larval prothoracic segment is proved by the fact that the first pair of stigmata becomes what authors call the “prothoracic” stigmata of the perfect insect. But Bugnion thinks that the projection which carries it, and which he calls the shoulder (Figs. 41 and 42), belongs to the mesonotum.
b. Appendages of the head
The antennæ.—These are organs of tactile sense, but also bear olfactory, and in some cases auditory organs; they are usually inserted between or in front of the eyes, and moved by two small muscles at the base, within the head. In the more generalized insects the antennæ are simple, many-jointed appendages, the joints being equal in size and shape. The antennæ articulate with the head by a ball and socket joint, the part on which it moves being called the torulus (Fig. 32, r). In the more specialized forms it is divided into the scape, the pedicel, and a flagellum (or clavola); but usually, as in ants, wasps, and bees, there are two parts, the basal three-jointed one being the scape, and the distal one, the usually long filiform flagellum. The antennæ, especially the flagellum, vary greatly in form in insects of different families and orders, this variation being the result of adaptation to their peculiar surroundings and habits. The number of antennal joints may be one (Articerus, a clavigerid beetle), or two in Paussus and in Adranes cœcus (Fig. 4312), where they are short and club-shaped; in flies (Muscidæ, etc.), they are very short and with few joints, and when at rest lying in a cavity adapted for their reception. In the lamellicorn beetles the flagellum is divided into several leaves, and this condition may be approached in the serrate or flabellicorn antennæ of other beetles. In Lepidoptera, and in certain saw-flies and beetles, they are either pectinate or bipectinate, being in one case at least, that of the Australian Hepialid (Abantiades argenteus), tripectinate (Fig. 44), and in the dipterous (Tachinid) genus Talarocera the third joint is bipectinate (Fig. 45). In Xenos and in Parnus they may be deeply forked, while in Otiocerus, two long processes arise from the base, giving it a trifid shape. In dragon-flies and cicadæ, they are minute and hair-like, though jointed, while in the larvæ of many metabolous insects they are reduced to minute three-jointed tubercles. In aquatic beetles, bugs, etc., the antennæ are short, and often, when at rest, bent close to the body, as long antennæ would impede their progress.
Fig. 43.—Different forms of antennæ of beetles: 1, serrate; 2, pectinate; 3, capitate (and also geniculate); 4–7, clavate; 8, 9, lamellate; 10, serrate (Dorcatoma); 11, irregular (Gyrinus); 12, two-jointed antenna of Adranes cæcus.—After LeConte. a, first joint of flagellum of antenna of Troctes silvarum; b, of T. divinatorius.—After Kolbe.
Fig. 44.—Tripectinate antenna of an Australian moth.