It is the same difficulty as appears with regard to the eyes of insects; we cannot understand how so small a nose can smell, any more than how such imperfect eyes can see so acutely. There are more things in Insect Physiology than can be explained by the physiology of the higher animals. Yet we must not ignore those physiological principles. Latreille* assigned the faculty of smell to the antennæ. But all analogy is against this conclusion. We may admit that the rule which requires a well-developed nose for a strong scent, and a well-developed brain for a strong intellect, and which expresses the points of a horse in the terms of anatomy, is not altogether applicable to insects; though we cannot tell exactly why. But the physical objections to his theory are very strong. Moisture seems an indispensable requisite for a part which is to appreciate odorous particles, and that is singularly wanting on the receiving surface of the antennæ.

Spence's supposition that there is a peculiar organ of smell inside the mouth, assigns the faculty, not to the mucous membrane lining the rest of the mouth, but to that lining the tracheæ.

* Newport, 'On the Antennæ,' sup. cit. p. 231.

THE thorax of insects presents to the entomologist a problem very like that which comparative anatomists have found in the vertebrate skull. Both these parts are composed of elements which appear in a simpler form in other parts of the body, in the rings of the abdomen and the spinal vertebræ respectively. The various forms, however, which the corresponding parts assume are not always easily referred to their original types. Their identification is often, indeed, a work of great difficulty. Excessive development in one, almost complete obsolescence in another, and fusion of distinct elements into a compact form in a third, produce forms in which none but the most skilled anatomists can clearly trace the homologies of insect or vertebrate structure.

Entomologists have the more difficult task in this analysis, on account of the minuteness of the subject of their examination, and of the want of much of that help which comparative anatomists obtain from observing the gradual development of

the different bones. And not a little additional difficulty has been thrown over the subject by confusion in the nomenclature of the various parts of the thorax. This difficulty pervades even the most elementary descriptions, the same word being often used in a different sense by different writers; and I can scarcely expect to have escaped altogether from this source of confusion, although it is only with the chief divisions, about which nearly all are agreed, that we are here concerned.

Fig. 2.-Diagram of the divisions of the thorax traced on its dorsal aspect.

The darkest lines indicate the divisions of the segments. The next shade denotes the subdivisions. The finest lines mark the boundaries of the yellow patches which are seen on the smaller wasps.

The first larval segment, as we have already seen, is absorbed in the formation of the head of the perfect insect. The thorax accounts for the three next segments, namely the second, third, and fourth, which constitute severally the pro-, meso-, and metathorax. Each of these segments is subdivided into four rings more or less distinct in different species, to which the names of præ-scutum and scutum, scutellum and post-scutellum have been given. And each of these has again been divided into a dorsal and a sternal, that is to say, as the insect stands on its legs, an upper, and a lower or lateral portion. Altogether, on the simplest mode of arrangement, there are twenty-four distinct pieces into which the insect

thorax is divided; and some anatomists count many more. The details of all these parts, particularly as displayed in the thorax of the Hymenoptera, have a special interest for entomologists. Our present field of observation, however, is much more limited, being almost exclusively confined to the dorsal surface, and to this, too, rather in a general than in a purely anatomical point of view. Indeed, the appendages of the thorax, the legs and wings, claim more of our attention than the skeleton.

The prothorax is the first division of the thorax, counting from before backwards. In the Hymenoptera this forms one compact piece, in which the lines of division of the component parts are not very easily distinguishable. The posterior boundary, however, is marked clearly enough on the dorsal surface, by two yellow lines which run backwards, diverging from behind the neck to the scale covering the root of the forewings. These yellow lines are the only distinctive marks which this division of the thorax displays in our British Vespa, though in some foreign genera, especially in Polistes, the whole of the dorsal portion of the prothorax is highly coloured. Both in the Social and Solitary group the prothorax is prolonged backwards to an unusual extent, apparently to give additional space within it for the front attachment of the depressor muscles of the wings. The portion of the prothorax which appears on the back represents the scutellum. The morphological nature of the loose ring or collar which surrounds the neck has been the subject of much discussion, but it is now pretty generally agreed that it represents the sternal portion of the thorax, and should,

strictly speaking, be called the prosternum. It carries the front pair of legs, which are the only appendages of this division of the thorax.

We must sacrifice many specimens to gain any clear idea of the arrangement of these parts. The lines of division between the different segments are more easily traced on the inner than on the outer surface, and in a dried than in a recent specimen. If we break a dried thorax carefully to pieces, we find the edges turned in to fit against each other along these lines of division. The first and last rings of each segment, in fact the præ-scutum and the post-scutellum, are, as a rule, thus turned in and do not appear at all on the surface. By the apposition of these flanges the connection is strengthened, ⚫ and a ridge is formed for the attachment of muscles on the inner surface.

The meso-thorax is chiefly represented by its scutum or second, as the pro-thorax is by its scutellum or third, component ring. The peculiar conformation of the back of the thorax of wasps is due to the development of the meso-scutum into a broad convex surface. It is black in the smaller wasps but variegated in the hornet. Behind it lies the scutellum, which is a transverse band, brown in the hornet, but in the wasps black, with the exception of a semicircular yellow spot at each end.

Not to map out the thorax with tedious minuteness, I would only indicate the chief divisions on its lateral aspect. As the divisions of the back are expressed by variations of the word scutum, so the sides have had the word sternum appropriated to them in nearly the same way: there is a pro-, meso-,