HOST RELATIONS 343 



Host Relations of Parasites 



The behaviour of entomophagous parasites in relation to their 

 hosts reveals many striking and remarkable features. As a general 

 rule their activities result in the death of the insects attacked, 

 which usually succumb very soon after the parasites have left 

 their bodies. In other cases the fatal effects of parasitisation 

 may reach this climax more slowly, and the host is capable of 

 limited reproduction before its premature death supervenes. This 

 regularly happens with the adult females of the brown scale 

 {Coccus caprece) when parasitised by certain Chalcids, while some 

 aphides are capable of restricted viviparous reproduction when 

 parasitised by Aphidius if they have passed the third ecdysis. 

 Instances are known of adult Coleoptera which have remained 

 active, and laid eggs, after larvae of the Braconid Dinocampus 

 have issued from their bodies. In this connection it is noteworthy 

 that Timberlake (1916) was able, under experimental conditions, 

 to rear two generations of D. americanus from a single individual 

 Coccinellid before death of the latter supervened. Such survivals 

 are probably exceptional and the result of relatively light parasi- 

 tisation affecting robust and well-developed host individuals. 

 Furthermore, the parasites feed upon the blood and fat-body, 

 sedulously avoiding injuring the more vital organs. Many 

 haemophagous or steatophagous parasites bring about the death 

 of their hosts largely on account of the nutritive drain which their 

 feeding activities entail upon the latter. In cases when the hosts 

 are adult insects, the partial or complete suppression of the 

 reproductive functions of the parasitised individuals (parasitic 

 castration) is a common feature. This occurs with the European 

 earwig when parasitised by the Tachinid Digonochceta setipennis, 

 with many Homoptera parasitised by Dryinidae and in both the 

 Homopterous and Hymenopterous hosts of Strepsiptera. As 

 previously mentioned, the activities of the Strepsiptera rarely 

 result in the death of the hosts, which continue to live and feed 

 after the male parasites have left their bodies. 



During their early life endoparasites lie within their hosts 

 surrounded by the blood or other serous fluid from which they 



