44 MISCELLANEOUS PAPERS. 



conditions, but its minute size renders it almost impossible to 

 determine this point. 



Samuel Doten ^ has recently published some interesting informa- 

 tion on the relation of food to the length of life of certain parasites 

 kept in confinement. He found that under favorable conditions some 

 species could be kept alive in confinement for months. 



CAUSES OF MORTALITY IN THE OPEN. 



Many of the adults of this tiny hymenopteron suffer an untimely 

 death, instances of which have been observed in the present investiga- 

 tion. It was a common sight to see two or three entangled in a spider's 

 web on the underside of a single leaf, and these invariably perished, 

 as they were too frail to extricate themselves. On one occasion the 

 larva of a syrphid was observed to seize one of these parasites and 

 kill it. The larva of Tnphleps insidiosus was also noted, in one 

 instance, with this insect impaled on its beak. 



One female of TJiripodenus russelli was observed on a croton leaf, 

 attempting oviposition in the larva of Heliothrips Tixmorrlioidalis . 

 It was frequently caught by the sticky excrement of its host, and 

 only mth great difficulty succeeded in freeing itself. On tlie next 

 morning this parasite was found fastened to the leaf and dead. Thus 

 the excrement of the larvae of Heliothrips may serve as a protection 

 to the thrips against its natural enemy. 



EFFECT OF OVIPOSITION IN DIFFERENT AGES OF HOST LARV.ffi. 



At the present time our observations show that the degree of 

 maturity of the host larvae bears some relation to the length of time 

 between oviposition by the parasite and indication of parasitism in 

 the host. In all cases observed where the parasite oviposited in 

 larvae that were nearly fully grown either the parasitic larvae did not 

 develop or the length of time required for their development was 

 lengthened a number of days. Also in one experiment where ovi- 

 position took place in thrips larvae only 2 days old the length of 

 time required until there were signs of parasitism was 14 days — 

 considerably more than the average. It appears that the best results 

 are obtained when the parasite selects host larvae about half grown 

 as its victims. 



LIFE HISTORY. 



The life history of Thripoctenus russelli is taken from records made 

 under artificial conditions, as the material was confined in vials 

 closed by cotton plugs. These were kept in an open-air insectary; 

 hence the other conditions were as nearly natural as it was possible 

 to make them. The fluctuations of temperature were recorded on a 



iTech. Bui. 78, Nevada Agr. Exp. Sta., September, 1911. 



