The structure of these insects is very remarkable: the typical genus _Stylops_ is named from its compound eyes, which consist of a very few (about fifteen) hexagonal facets, seated upon a sort of footstalk. The mandibles are lancet-shaped and very acute, and the head, by reason of the protuberant eyes, has very much the shape of a dumb-bell. The antennæ are branched, but in _Halictophagus_, they are flabellate. The thorax is greatly developed; the superior wing is like a rudimentary wing-case, and is twisted, the inferior wings are very large, and fold along the abdomen in repose like a fan; the legs are slender, and the tarsi with four joints in _Stylops_, with three in _Halictophagus_, and with two in _Elenchus_; the abdomen is long, very flexible, and consists of eight segments. The insects themselves do not exceed a quarter of an inch in length in the largest, but they are generally very much smaller. The perfect insect is very short-lived, not surviving many hours, as just stated. They are usually found in the months of May and June, and they have been discovered to infest several species of _Andrena_ and _Halictus_, for instance the _A. nigro-ænea_, upon which Mr. Kirby first found it; _A. labialis_, which I have frequently caught stylopized; _A. rufitarsis_, _fulvicrus_, _Mouffetella_, _tibialis_, _Collinsonana_, _varians_, _picicornis_, _nana_, _parvula_, _xanthura_, _convexiuscula_, _Afzeliella_, _Gwynana_, etc., and upon _Halictus æratus_, etc. The other mode of parasitism destructive to the bees is where the parasite deposits its own egg upon the provender stored by the bee for the sustenance of its own young. The young of the parasite, either by being more speedily hatched or more rapacious than the larva of the sitos, starves the latter by consuming its food. This kind of parasites consists of several _Diptera_, but they are mostly bees which form a distinctive subsection of the family of true bees (_Apidæ_), the subsection being called the _Nudipedes_ or naked-legged, from their not having the necessary apparatus of hair upon the posterior thighs or shanks, for the conveyance of pollen wherewith to store their nests. Thus nature, having rendered them unable to perform this duty to their offspring, has imposed upon them the necessity of resorting to strangers to support them, and they are not led to it by idleness or indifference. These insects consist, with us, of six genera, the species of which are individually attached to some particular bee, who thus nurtures their young. They are, as a rule, gayer insects than those which they infest, and the genus most abundant in species is _Nomada_, which attaches itself chiefly to _Andrena_, although some of its species, especially the smaller ones, infest the species of _Halictus_, and one frequents _Eucera_. _Melecta_ appears confined to _Anthophora_; _Epeolus_ to _Colletes_; _Stelis_ perhaps to _Osmia_, judging from the great