l8 CONNECTICUT GEX>L. AND NAT. HIST. SURVEY. [BuU. 



Siphonaptera). The bot-flies (order Diptera) are internal para- 

 sites, the eggs being laid upon hairs and taken into the stomach 

 of the host through the mouth ; the larval stage is passed in the 



host. 



Many four-winged flies (order Hymenoptera) and two- 

 winged flies (order Diptera) are parasitic upon other insects. 

 The egg is usually laid on or in the body of the larva of the 

 host; and, after hatching, the young grubs or maggots feed 

 therein until ready to transform, the host usually living just 

 long enough to mature the parasites. By far the greater part of 

 these insects are of small size and belong to a group commonly 

 known as parasitic Hymenoptera. 



In nature we find a balance between host and parasite. A 

 certain species may, on account of abundance of food, become 

 prevalent, and, if parasites are scarce, it will increase in numbers 

 rapidly; but, when it becomes prevalent, it in turn furnishes 

 for the parasites an abundant supply of food, which, other things 

 being equal, enables them to multiply rapidly and soon over- 

 take their host in point of numbers and finally subdue it. The 

 host will then be scarce for a period, and on account of lack of 

 food the parasites must of necessity be kept in check until again 

 the host becomes abundant, when a corresponding increase in 

 parasites will follow. Thus host and parasites play a veritable 

 ;game of see-saw with each other in scarcity and abundance. 

 Under these conditions no species is allowed to dominate the 

 earth, but in the long run every species is kept in check by 

 natural enemies. 



The illustrations here given refer only to simple parasitism. 

 In nature the case is usually much more complex, because the 

 primary parasites are attacked by other parasites which are 

 known as secondary parasites. The latter in turn may be 

 parasitized by tertiary parasites, and so on. The term hyper- 

 parasite includes all except the primary parasite. 



Nevertheless, in spite of the complexity, nature is able to 

 preserve a sort of balance between her various organisms. It 

 is only when nature's balance is disturbed that we find certain 

 species overrunning the earth and crowding out others. Man 

 has been the disturbing factor, by cutting off the forests and 

 native vegetation, thus destroying food supply ; by growing large 



