The Vagaries of Colour and Structure in the Sexes
As a rule the male is rather smaller and especially slenderer than the female, but there are notable exceptions; in one genus of the fossors, Myrmosa for instance, the male is many times larger than the female. In this case the male is winged and the female is wingless. Also, if there is a difference in brightness of coloration between the sexes, as a rule the male is duller than the female—this is especially the case among the bees—but if there is any eccentricity in the form of the limbs it is almost sure to occur in the male, and I think one would not go far wrong in saying that when peculiar features occur in the female, the reason for them is more or less apparent, whereas for the eccentricities of the male there really often seems to be no assignable cause. These male eccentricities are often exceedingly marked. A very good example of them occurs among the small “keyhole” wasps. All the British species are practically alike in coloration. They may vary in having a greater or less number of yellow bands on the body, but otherwise their distinctions rest on structure. In the females the antennæ are slightly thickened towards the apex, but otherwise they are simple. The males, however, are divided into three quite distinct groups. In the first of these, the end joints of the antennæ are rolled up in more or less of a spiral (fig. 23, 2); in the second, the apical joint is turned sharply back like a hook (fig. 23, 1); in the third, the end joints of the antennæ are simple and more or less like those of the female. Now if we examine the legs of the males in the first group we shall find still greater peculiarities; in two of our species there is a long yellow spine at the extreme base of the middle leg on the little joint by which it articulates on to the body (fig. 24, 2), and a curious pencil of hairs on each side of the mouth. In two others, the femora, or thighs of the middle legs, are cut into two deep somewhat semicircular incisions (fig. 24, 1)—a most curious character; but here again the females have no corresponding peculiarities. There seems to be no explanation known for these vagaries, and yet one feels that there must be some object served by them. If we turn to the bees we shall find that in many species the face of the male is white to a greater or less extent, whereas that character is very rare in the female. The front feet are produced into a wide flattened form in some, in others the middle legs are extraordinarily developed, and provided with tufts of hairs, etc. Another form of male development lies in the form of the head. This is sometimes very much enlarged—often varying considerably in this respect in specimens of the same species; there is often a projecting tooth or spine on the mandible or jaw at its base, or frequently on the cheek just above it. Then in the fossors the males of the genus Crabro break out into numerous eccentricities; in some, two or more of the joints of the antennæ are soldered together and curved or cut out into curious forms (fig. 26); in others the front shin or tibia is formed like a concave shield or shell (fig. 25), and all the joints of that leg more or less distorted; in another male (a rather doubtful native which has not been taken in this country for fifty years) the head is narrowed behind into an almost ridiculously small neck, being quite triangular in form, viewed from above, with the eyes projecting from its anterior angles (fig. 27, 1), the female head being of normal form (fig. 27, 2).
In the males of several species of fossors and bees the eyes are enormously developed, joining one another on the top of the head. This condition occurs also in the drone of the hive bee. The male of Astatus, which has this character, has also a peculiar habit. It sits basking in the sun on some bare sandy spot, and when disturbed makes a sort of circular detour and pitches down again exactly on the spot from which it started up. An increased length of the antennæ is another male characteristic. This is carried to an extraordinary development in what is called the “long horned bee”; this bee, which is pretty common in some places, has antennæ which, when directed backwards, are almost as long as its body—the female has quite an ordinary pair.
Another set of male characters which are of great value to systematists lies in the hidden apical segments of the underside; although these are hidden, being telescoped up inside the segments which close the apical opening of the body, they often assume most curious and beautiful forms, and are characters whereby the males of a species may be determined with certainty when the females defy all one’s endeavours to discover their identity.