WORMS 185 



A. Spiralis, but various invertebrates such as Gammarus pulex may be 

 used as host. In the case of the well known gizzard worm, Acuaria 

 hamulosa, which is generally located near the opening between the 

 stomach and intestine, chickens become infested by eating various 

 insects, like weevils and grasshoppers. 



The blood of many wild birds is found to be teeming with larval 

 Spiruroids known as microfilariae. In Britain these larvae have been 

 recorded from blackbirds and thrushes and the rate of infection was 

 said by Coles to be very high. In the United States 60 per cent, of a 

 population of wild crows was found to be infected with microfilariae 

 The adult worms live in the connective tissues or body cavities of the 

 host. The female gives birth to free-living embryos, the microfilariae, 

 which swarm in the blood where they await ingestion by a blood- 

 sucking insect, which, in the case of the species infecting man, is a 

 mosquito. Inside the intermediate host they undergo further develop- 

 ment and, after a certain period, assemble in the proboscis of the mos- 

 quito. During the insect's next blood meal they break loose from the 

 mouth-parts and creep out on to the skin of the host. They quickly 

 penetrate through the mosquito " bite " or any other abrasion and by an 

 unknown route return to the original site of infection. In certain of the 

 species infecting man there is a diurnal periodicity in the appearance of 

 the microfilariae in the peripheral blood stream. During the day 

 scarcely any are present, but at night between 10 p.m. and 4 a.m. they 

 teem near the surface of the body. Nobody has so far discovered what 

 mysterious influence drives them outwards, but as certain species of 

 mosquito only bite at night, it has been suggested that this is an adjust- 

 ment of the life-cycle which brings the larvae into contact with these 

 insects. There, while their host sleeps, they wait like expectant 

 lovers. 



Some ingenious person has, with the aid of a microscope, watched 

 microfilariae in the transparent web of a frog's foot. It was seen that 

 they work up the capillaries against the blood stream and are apparently 

 actively attracted to the saliva of the insect vector, which it pumps into 

 the wound at the moment of biting. In the case of Onchocerca — a mam- 

 malian Spiruroid carried by blackfly — the microfilariae swarm im- 

 mediately below the epidermis. 



The life-cycles of the numerous species from birds are not known, 

 but as those from man, frogs and lizards are carried by species of gnats 

 {Culex) it is highly probable that they follow a similar course in avian 



