A difficulty similar to what is found in the distinction between Andrena and Cilissa, arises in the separation of Chelostoma from Heriades, and which we shall again meet with in drawing the line between Anthophora and Saropoda. The difference can only be detected by examining the trophi, but a pin and a little patience will elucidate the separation. The males in all but two species of Anthophora may be readily associated with their partners; but in these two the females are entirely black, and so hirsute as to have led Ray (wanting the knowledge of the use of the trophi and posterior shanks) to unite the one he knew with his Bombylii; their males are fulvous, and the latter have a remarkable elongation of the intermediate tarsi, from one of the joints of which also a tuft of hair or a loose lateral fringe projects, giving them thus a wider expansion, and the use of which is prehensile, the same as that for which the anterior tarsi in some of the Megachiles and in our single Anthidium receive their dilatation. This structure has also the effect of adding very considerably to the elegance of their appearance when they are in fine condition.
The male Apathi can only be distinguished from the male Bombi by familiarity with specific characteristics, or by the examination of the trophi. But the former is the more certain mode of separation, as the trophi in Bombus vary in some species, but not sufficiently to authorize generic subdivison. General appearance will mark where they approximately belong. The length of their antennæ sufficiently distinguishes them as males, and they may be taken with impunity in the fingers from flowers for examination, being, like all the male aculeate Hymenoptera, unarmed with stings. The female Apathi may be superficially distinguished from the female Bombi, which they most resemble, exclusively of the generic characters of the convex and subpubescent external surface of the posterior tibiæ and the trophi, also by their abdomen being considerably less hirsute than that of the genuine Bombi, in which it is entirely covered with dense shaggy hair, whereas in Apathus there is a broad disk upon its surface nearly glabrous. If I remember rightly, it is the male Apathi only, and not the male Bombi, which emit on capture a pleasantly fragrant odour of attar of roses.
The table will suffice for distinguishing the male Apis from all other male Apidæ, and which has a further peculiarity exhibited by no other of our native bees, in the conjunction upon the vertex of the compound eyes, in front of which, upon the frons, the simple eyes or ocelli are placed in a very slightly-curved line.
These indications are enough to enable the beginner to work his way smoothly, and a little practice will soon render these observations superfluous.
The economy of nature is so perfect that wherever we can trace a difference, we may assume that a reason and a purpose exist for the variation. Thus we do not know why some bees have three submarginal cells to their wings, and others only two. Nor do we know what governs their variety of shape. The deficiency we might think implied inferiority; but this cannot be, for those with most frequently the smaller number, viz. the artisan bees, are, in the majority of cases, the most highly endowed, and have the most special habits.
In the relative numbers of the maxillary and labial palpi, there are remarkable differences, the reason for which we cannot trace, for, as before observed, we do not know even their function, which would perhaps guide us to other views. Their normal numbers are six maxillary, and four labial palpi. The latter take remarkable relative development and peculiarity of insertion and form, especially in the Apidæ; but throughout the whole series of our bees, they are never reduced to fewer than their normal number, whereas the maxillary palpi never have similarly large development of structure, and are variously modified in number and consistency from the typical or normal condition.
Thus in Eucera and Melecta there are but five joints; in Osmia and Saropoda, four; in Chelostoma and Cœlioxys, three; in Anthidium and Megachile, etc., two; and in Epeolus and Apis but one.
In this collocation no incidental peculiarity beyond diversity is apparent, for in the first instance a parasite and a bee not parasitical are associated; and in the last, a parasite is associated with the bee which has the most elaborate economy, and the most largely developed instinct of all known insects. Nor are, in any case, those parasites associated by these means with their own sitos, or insect upon which they are parasitical.
Thus encouragement attends the beginner at the very outset of his study; and the prospect of a wide field for discoveries, in many directions, lies open to him, to excite his curiosity and to stimulate his industry to the pursuit of higher aims than the mere accumulation of species.