The genus _Andrena_ includes a great number of species, Britain possessing about fifty. They may be described in a general manner as Insects much resembling the honey-bee—for which, indeed, they are frequently mistaken—but usually a little smaller in size. Many of the bees we see in spring, in March or April, are of this genus. They live in burrows in the ground, preferring sandy places, but frequently selecting a gravel path as the locality for their operations; they nearly always live {26}in colonies. Great difficulties attend their study on account of several points in their economy, such as, that the sexes are different, and frequently not found together; also that there may be two generations of a species in one year, these being more or less different from one another. Another considerable difficulty arises from the fact that these bees are subject to the attacks of the parasite _Stylops_, by which their form is more or less altered. These Insects feed in the body of the bee in such a way as to affect its nutrition without destroying its life; hence they offer a means of making experiments that may throw valuable light on obscure physiological questions. Among the effects they produce in the condition of the imago bee we may mention the enfeeblement of the sexual distinction, so that a stylopised male bee becomes less different than it usually is from the female, and a stylopised female may be ill developed and less different than usual from the male. The colours and hair are sometimes altered, and distortion of portions of the abdominal region of the bee are very common. Further particulars as to these parasites will be found at the end of our account of Coleoptera (p. 298). We may here remark that these _Stylops_ are not the only parasitic Insects that live in the bodies of Andrenidae without killing their hosts, or even interrupting their metamorphoses. Mr. R. C. L. Perkins recently captured a specimen of _Halictus rubicundus_, from which he, judging from the appearance of the example, anticipated that a _Stylops_ would emerge; but instead of this a Dipterous Insect of the family Chloropidae appeared. Dufour in 1837 called attention to a remarkable relation existing between _Andrena aterrima_ and a parasitic Dipterous larva. The larva takes up a position in the interior of the bee's body so as to be partly included in one of the great tracheal vesicles at the base of the abdomen; and the bee then maintains the parasite in its position, and at the same time supplies it with air by causing two tracheae to grow into its body. Dufour states that he demonstrated the continuity of the tracheae of the two organisms, but it is by no means clear that the continuity was initially due to the bee's organisation. [Illustration: Fig. 13—Parasitic Dipterous larva in connection with tracheal system of _Andrena aterrima_. (After Dufour.)] {27}[Illustration: Fig. 14.—_D. hirtipes_ ♀. Britain.]