In the _Neuroptera_, the _Libellulidæ_ are remarkable for the differences of colour in the sexes. In the common _Libellula depressa_, which you may see hawking over every pool, the abdomen of the male is usually slate-colour, while that of his partner is yellow, but with darker side-spots. Reaumur, however, noticed some males that were of the same colour with the females[708]. Schelver observed, when he put the skins of _Libellula depressa_ into water, that the colours common to both sexes were in the substance of the skin, and remained fixed; while those that were peculiar to one could be taken off with a hair-pencil, and coloured the water: which therefore were superficial, and, as it were, laid on[709]. The yellow males, therefore, that Reaumur observed, were probably such as had the superficial blue colour which distinguishes them washed off. In _Calepteryx Virgo_ Leach, the former are of a lovely silky blue, and the latter green. In _Agrions_ F. nature sports infinitely in the colours of the sexes.
In the order _Hymenoptera_ there are often differences equally great; the sexes of many of the Ichneumons and Saw-flies are of quite different colours. The former tribe Linné has divided into sections, from the white annulus observable in the antennæ of some, and from the colour of their scutellum: but these are often merely sexual characters[710]. The male of _Anthophora retusa_ Latr., a kind of wild bee, is wholly black, the female wholly gray, and of so very different an aspect that they were long regarded as distinct species; a mistake which has likewise occurred with regard to the sexes of _Osmia cærulescens_, another bee, of which the male has a bronzed and the female a violet abdomen[711]. The nose of male _Andrenæ_ Latr. is often yellow, or white, as in _A. hæmorrhoidalis_--when that of the female is black[712]. The _labrum_ also is often of a different colour in the sexes, as in _Ceratina_ Latr.
In the _Diptera_, _Aptera_, _Arachnida_, &c., I am not aware of any striking difference in the colours of the sexes.
ii. The sexes of insects vary (but more rarely than in colour) in their _sculpture_ also, and _pubescence_. Thus the elytra of the females of many of the larger water-beetles (_Dytiscus_) are deeply furrowed, while those of the males are quite smooth and level[713]. The thorax of the female in several species of _Colymbetes_ of the same tribe, as _C. Hybneri_ and _transversalis_, on each side has several tortuous impressed lines or scratches, like net-work, which are not to be discovered in the male. _Hyphydrus gibbus_ Latr., which differs solely from _H. ovalis_ (_Dytiscus ovalis_ Illig.) in being thickly covered with minute impressed puncta, is, from the observation of the Rev. R. Sheppard, the other sex of this last, with which he has taken it coupled; and it is by no means improbable that _Hydroporus picipes_ (_Dytiscus punctatus_ Marsh.) and _H. lineatus_,--between which, as Gyllenhal has justly observed, the same difference only exists,--are in like manner sexual varieties. With respect to _pubescence_, I have not much to say. Another aquatic beetle, _Acilius sulcatus_ Leach, has not only its elytra sulcated, but the furrows of these, and a transverse one of the thorax, are thickly set with hair; while the male is smooth, and quite naked. Particular care seems to have been taken by the Creator, that when all the above inhabitants of the water are paired, the male should be able to fix himself so firmly, by means of his remarkable anterior tarsi, (which I shall afterwards describe,) and these asperities, &c. in the upper surface of his mate, as not to be displaced by the fluctuations of that element, the reluctance of the coy female, or any other slighter cause.