No. 637] 



PARASITISM 



153 



viduals below the surface. Even memory could easily 

 play a part liere, if the host were submerged during the 

 development of the parasite, and the latter emerged as 

 an adult below the water, from which it must escape by 

 locomotion through the water. 



In the case of aquatic Hymenoptera, it is readily seen 

 that we can not correlate taxonomy with habits accord- 

 ing to any generalized scheme, although the several 

 genera show structural characters associated with their 

 unusual habits. Most striking is the number of Myma- 

 ridse and TrichogrammatidaB included, minute insects 

 whose wings are naturally well suited for swimming. 



Frequently a secluded habitat acts as a powerful fac- 

 tor in restricting the kinds of parasites that can attack 

 certain types of hosts. Thus, wood-boring insects can 

 be reached only by species provided with long oviposi- 

 tors. Such restrictions are clearly defined and many 

 other examples might be cited. Partial inaccessibility 

 of the host may even occur in the case of parasites other- 

 wise well suited to their host, as for example in the case 

 of a (''oiinuoii ])nrasito of the i'g\ii's of the gipsy moth, 

 wliich is able to oviposit only m tho eggs occupying a 

 su|-»(M-rK'ial ])o>ilioii in \ho o,u\ii--mass of the host. Some- 

 times dillicultios may bo overcome by the presence of an 

 active first-stage larva. This may exhibit most extraor- 

 dinary behavior as has been described by Smith ('17) 

 in the Chalsis-fly, Pcrilampus. Here the Perilampus 

 egg is deposited and hatches away from the body of the 

 host as a ])laniaiiim whieli later attaches itself to the 

 host and roinains there till the host completes its growth, 

 after which iho planidium begins its parasitic life. A 

 second species tlint is a liypcrparasite seeks out the pri- 

 mary parasite in the rnt( r[)illar host through which it 

 bores its way and ilu iv awaits the exit of the primary 

 parasite ])et'oiv proceeding ^vith its development. 



Ao-ain, thr I'cmale of some .\u;u--parasites attach them- 

 selves to individuals of the host species and are thus 

 carried to the place where the eggs within which they will 



