338 PARASITISM 



unknown among Tachinids and rare in other parasitoidal Diptera,-^ 

 a feature which contrasts with its prevalence among Hymenoptera. 

 Adaptations to an exclusively ectoparasitic life of this kind are 

 not markedly different from those exhibited by most non-parasitic 

 Hymenoptera, whose larvae likewise pass an inert existence in 

 contact with ample nutriment. In view of the uniformity of 

 conditions prevailing during the whole parasitic life, it is scarcely 

 surprising that the metamorphoses of such ectoparasites do not 

 involve the striking diversity of larval forms so frequent among 

 the endoparasitic members of the same order. 



Upon emergence from the egg the ectoparasitic larva punctures 

 the skin of its host with its mandibles and imbibes the body- 

 fluids through the rupture thus formed. It rarely moves from 

 the position originally taken up, and this method of nutrition 

 commonly prevails throughout the larval existence. In cases 

 where predatory instincts subsequently develop the larva may 

 become more active and finally consume its host bodily. 



Structurally, ectoparasites present few constant features 

 which are unquestionably correlated with their mode of life 

 and at the same time undeveloped in some or other of the 

 endoparasitic forms. The most important character is afforded 

 by the respiratory system and, as a rule, open spiracles are present 

 from the first instar onwards. In some cases, as in Aphelinus 

 mytilaspidis, for example, the full complement of spiracles is 

 present at the time of eclosion from the egg (which never obtains 

 in endoparasitic Hymenoptera), in others it is not acquired until 

 later. It should be pointed out, however, that information 

 respecting the tracheal system of the early instars is very 

 incomplete and generalisations relating thereto may require 

 subsequent modification. In the primary larvae of many 

 ectoparasites the head is relatively larger and more firmly 

 sclerotised and the mandibles more powerful than in subsequent 

 instars, characters which are correlated with the maintenance of 

 a firm hold upon the host and with the initial perforation of the 

 integument. The body frequently exhibits a definite chsetotaxy, 



1 Instances of this type of life-cycle are found in the Bombyliidae among 

 Diptera. 



