These insects in their perfect state vary very considerably in size, both individually and specifically, the former depending upon both the quantity and quality of the food stored up, for the pollen of different plants varies possibly in its amount of nutriment, else why should we observe so marked a difference in the sizes of individuals whose parent instinct would prompt to furnish them with an uniform and equal supply. The differences of specific appearance is often very considerable in long genera, and perhaps in no genus is it more conspicuously so than in _Andrena_, for here we have some wholly covered with dense hair, and others almost glabrous; others again with the thorax only pubescent; some are black, some white, some fulvous, or golden tinted, and some red; some we find banded with decumbent down, and others with merely lateral spots of this close hair, but the most prevalent colour is brown, which will sometimes by immaturity take a fulvous or reddish hue. In many males we see excentrically large transversely square heads broader than the thorax, which also have widely spreading forcipate mandibles, with often a downward projecting spine at their base beneath; and it is chiefly these extravagantly formed males which are most dissimilar to their own partners that the result of observation alone confirms their specific identity. In other cases the males are so like their females that a mere neophyte would unite them. In many males the clypeus and labrum are white, which also occurs in some females; for instance, in _A. labialis_, but this peculiarity is found more rarely in this sex. The species are much exposed to the restricting influences of several parasites, whose parasitism is of a varying character, but the term should properly be applied only to the bees which deposit their eggs in their nests, and whose young, like that of the cuckoo among the birds, thrives at the expense of the young of the sitos by consuming its food, and thus starving it. These parasites consist of many of the species of _Nomada_, very pretty and gay insects, but in every case totally unlike the bee whose nest they usurp. Several of the species of these _Nomadæ_ are not limited to any particular species of _Andrena_, but infest several indifferently, whereas others have no wider range in their spoliation than one single species, to which they always confine themselves. In my observations under the genus _Nomada_ I shall notify those which they assail amongst the _Andrenæ_, as well as the other genera which they also infest.