ENTOMOLOGY

SANITARY ENTOMOLOGYswallow the eggs and if they are suitable intennediate hosts for theparasites the young worms go through several developmental stages andfinally within the bodies of the insects reach a stage in which they areready to be introduced into tIte body of the final host. Certain parasiteswhose adult stages live in relation with the blood vessels of the final hostdischarge their young into the blood stream whence they may be ingestedby bloodsucking insects in whose bodies they undergo development to astage infective for the final host. Aquatic insects may swallow free-livinglarval stages of parasites, or may be actively attacked by larval parasiteswhich gain entrance to their bodies by penetrating the cuticle.These insects may in turn be eaten by other insects and the infection thuspassed on -to them.In some cases the parasites may be taken up by insects or entertheir bodies during an early stage of development of the insects andpersist in later stages. Infection may thus occur during one stage of theinsect but the development of the parasite to a stage infective for thefinal host may not be completed until after the insect has reached a laterstage. Thus flies become infected with a certain parasite of the horseduring the maggot stage, but the young pa~asites do not become sufficientlydeveloped to be returned to the final host until the flies havereached the pupal or adult stage.MODE OF INFECTION OF VERTEBRATE HOSTSParasitie worms that have insects for intermediate hosts reaehtheir final hosts in various ways. In the case of some species the insecthosts are swallowed either as the habitual food of the final hosts, orincidentally with food or drink. In other instances the young worm mayhave already escaped from its insect host before it is taken in with foodor drink by its final host. The cases of accidental infection with horsehairworms not normally parasites of human beings are likely to havehappened in this way. The parasites of which bloodsucking insects areintennediate hosts may be introduced into their final hosts as a result ofthe escape of the larval parasites from the insects at ,a time when theinsects are drawing blood. Commonly the larvre burst through a lveakspot in the cuticle of the insect and then burrow into the skin of thefinal host.SPECIES OF WORl\IS FOUND IN INSECTSThe parasitic worms of the higher animals in whose life history insectsand insect-like organisms play a part, belong to two large zoologicalgroups, Plathelminthes and Nemathelminthes. The former may be subdividedso far as concerns parasitic forms into Cestoda, or tapewonns,