52 SANITARY ENTOMOLOGY 



swallow the eggs and if the}- are suitable intermediate hosts for the 

 parasites the young worms go through several developmental stages and 

 finally within the bodies of the insects reach a stage in which they are 

 ready to be introduced into the body of the final host. Certain parasites 

 whose adult stages live in relation with the blood vessels of the final host 

 discharge their young into the blood stream whence they may be ingested 

 by bloodsucking insects in whose bodies they undergo development to a 

 stage infective for the final host. Aquatic insects may swallow free-living 

 larval stages of parasites, or may be actively attacked by larval para- 

 sites which gain entrance to their bodies by penetrating the cuticle. 

 These insects may in turn be eaten by other insects and the infection thus 

 passed on to them. 



In some cases the parasites may be taken up by insects or enter 

 their bodies during an early stage of development of the insects and 

 persist in later stages. Infection may thus occur during one stage of the 

 insect but the development of the parasite to a stage infective for the 

 final host may not be completed until after the insect has reached a later 

 stage. Thus flies become infected with a certain parasite of the horse 

 during the maggot stage, but the young parasites do not become suffi- 

 ciently developed to be returned to the final host until the flies have 

 reached the pupal or adult stage. 



MODE OF INFECTION OF VERTEBRATE HOSTS 



Parasitic worms that have insects for intermediate hosts reacli 

 their final hosts in various ways. In the case of some species the insect 

 hosts are swallowed either as the habitual food of the final hosts, or 

 incidentally with food or drink. In other instances the young worm may 

 have already escaped from its insect host before it is taken in with food 

 or drink by its final host. The cases of accidental infection with horse- 

 hair worms not normally parasites of human beings are likely to have 

 happened in this way. The parasites of which bloodsucking insects are 

 intermediate hosts may be introduced into their final hosts as a result of 

 the escape of the larval parasites from the insects at a time when the 

 insects are drawing blood. Commonly the larvae burst through a weak 

 spot in the cuticle of the insect and then burrow into the skin of the 

 final host. 



SPECIES OF WORMS FOUND IN INSECTS 



The parasitic worms of the higher animals in whose life history insects 

 and insect-like organisms play a part, belong to two large zoological 

 groups, Plathelminthes and Nematliclminthes. The former may be sub- 

 divided so far as concerns parasitic forms into Cestoda, or tapeworms, 



