_Habronema muscæ_ is a worm which has long been known in its larval stage, as a parasite of the house-fly. Carter found them in 33 per cent of the house-flies examined in Bombay during July, 1860, and since that time they have been shown to be very widely distributed. Italian workers reported them in 12 per cent to 30 per cent of the flies examined. Hewitt reported finding it rarely in England. In this country it was first reported by Leidy who found it in about 20 per cent of the flies examined at Philadelphia, Pa. Since then it has been reported by several American workers. We have found it at Ithaca, N. Y., but have not made sufficient examinations to justify stating percentage. Ransom (1913) reports it in thirty-nine out of one hundred and thirty-seven flies, or 28 per cent. [Illustration: 123. An Echinorhynchid, showing the spinose retractile proboscis.] [Illustration: 124. June beetle (Lachnosterna).] [Illustration: Larva] Until very recently the life-history of this parasite was unknown but the thorough work of Ransom (1911, 1913) has shown clearly that the adult stage occurs in the stomach of horses. The embryos, produced by the parent worms in the stomach of the horse, pass out with the feces and enter the bodies of fly larvæ which are developing in the manure. In these they reach their final stage of larval development at about the time the adult flies emerge from the pupal stage. In the adult fly they are commonly found in the head, frequently in the proboscis, but they occur also in the thorax and abdomen. Infested flies are accidentally swallowed by horses and the parasite completes its development to maturity in the stomach of its definitive host. _Gigantorhynchus hirudinaceus_ (= _Echinorhynchus gigas_) is a common parasite of the pig and has been reported as occurring in man. The adult female is 20-35 cm. long and 4-9 mm. in diameter. It lacks an alimentary canal and is provided with a strongly spined protractile rostrum, by means of which it attaches to the intestinal mucosa of its host. The eggs are scattered with the feces of the host and are taken up by certain beetle larvæ. In Europe the usual intermediate hosts are the larvæ of the cockchafer, _Melolontha vulgaris_, or of the flower beetle, _Cetonia aurata_. Stiles has shown that in the United States the intermediate host is the larva of the June bug, _Lachnosterna_ (fig. 124). It is probable that several of the native species serve in this capacity. A number of other nematode parasites of birds and mammals have been reported as developing in arthropods but here, as in the case of the cestodes, experimental proof is scant. The cases above cited are the better established and will serve as illustrations. CHAPTER VIII ARTHROPODS AS ESSENTIAL HOSTS OF PATHOGENIC PROTOZOA MOSQUITOES AND MALARIA