A similar protandry exists in the parasitic forms of _Allantonema_,[189] of which there are several species—_A. mirabile_ in _Hylobius pini_, _A. sylvaticum_ in _Geotrupes sylvatica_, _A. diplogaster_ in _Tomicus typographicus_; but in their case the male and female forms which leave their host pair in the damp earth and give rise to larvae which make their way into the body of the beetle-grubs. Here they undergo very extensive retrogressive change. The body of the female, which becomes the shape of a thick sausage, is encapsuled and surrounded by a curious hypertrophied network of tracheae (Fig. 74). As is usually the case with the degenerate parasitic forms, there are practically no organs but the ovary, and this is {153}embedded in a fatty parenchyma which fills all the space within the skin. [Illustration: FIG. 76.—Four stages in the life-history of _Sphaerularia bombi_ Dufour, ♀. (From Leuckart.) A, Beginning of the protrusion of the uterus (_b_), × 66; B, later stage, × 66; C, later stage, × 12; D, the protrusion is complete, × 6. In each case _a_ represents the Nematode, and _b_ its protruded uterus.] _Atractonema gibbosum_, which lives in the body-cavity of the larva of _Cecidomyia pini_, has a similar life-history, but the parasitic form has a structural peculiarity which merits attention (Fig. 75). At the time of sexual maturity a swelling, which is caused by the prolapsus of the uterus and vagina, appears at the posterior end of the body; this swelling increases until it equals the rest of the body of the Nematode in size. Even this is far surpassed by a similar protuberance in _Sphaerularia bombi_, where the evaginated sac grows with such extreme rapidity that in a few weeks its length increases from .25 mm. to 15 mm. and its volume 60,000-fold, the increase being due, according to Leuckart, to the increase in size of the individual cells and not to their multiplication. The Nematode which has produced this enormous growth gets relatively smaller and smaller, and ultimately drops off (Fig. 76). The sexual larvae which arise from the eggs in this sac leave the body of the bee in which this species is parasitic by the anus, {154}and may live in damp earth, moss, etc., for months without taking nourishment, until the autumn, when they become sexually mature and, according to Leuckart, pair. The fertilised female is believed to bore her way into the humble-bee whilst the latter is seeking her underground winter quarters; this accounts for the fact that only queen bees are infected. The parasite is widely distributed both in Europe and North America; it is found in many species of _Bombus_, but most frequently in _B. lapidarius_ and _B. terrestris_. The presence of the _Sphaerularia_ affects the reproductive organs of the host, and reduces their fertility, so that an infected queen bee never succeeds in forming a colony. VI. FAMILY ANGUILLULIDAE.