HABITS OF TWO PAllASITES OF BLOW-FLIES. 



211 



and was trying to squeeze and crush the egg*. As soon as this 

 ceases it hastily disappears into the soil or carrion, apparently 

 stimulated by its experience to pupate. The foregoing obser- 

 vation, naturally, only refers to larvae free of the carrion or on 

 top of it (otherwise they could not have been observed), but those 

 attacked which are partially buried in it do not appear to behave 

 in the same manner ; they are surrounded by food and hidden 

 from the light- 



The first 20 -30 larvte attacked will be " j^aralyzed " instantly, 

 and then the poison apparently becomes less rapid in its effect ; 

 whilst it has been noticed that a female which rests for some 

 time and feeds, seems to renew the effectiveness of its poison. 



An individual female — as has been already noted—does not 

 normally attack the same larva twice. In this connection a 

 series of experiments were carried out under conditions as nearly 

 natural as was possible, and by employing one female at a time 

 it was observed that each larva parasitized, either free of the 

 carrion or partially embedded in it, made every endeavour to get 

 into the soil after it had recovered from its " temporary paralysis " 

 either directly or through tlie carrion. It appears to be definite 

 that effective ovi position stimulates the larva to pupate. How- 

 ever, in cases in which two or more females inject the sa.me larva — 

 the second one coming upon it just when it begins to move 

 or encountering it on its wa,y to escape, — its death ensues within 

 24 to 48 hours. Its organs appear to disintegrate, the dead 

 larva gradually darkens until it turns black; dissections disclosed 

 little else than a thick dirty putrid liquid. In warm weather, 

 with a temperature of about 22° C, the dead larva dries up 

 within 6 or 7 days. The majority of these over- parasitized larvsa 

 never succeed in getting into the soil. 



A female does not attack a motionless larva, but may prod it 

 with the ovipositor, usually causing some movement with 

 unpleasant results for tlie larva. It is also in this way that a 

 larva just recovering from "temporary paralysis" becomes 

 a victim to over-parasitism. 



On several occasions the writer has observed a moving larva 

 stop suddenly and lie motionless upon the approach of a female, 

 although it may have been touched by no more than one leg of 

 the latter. This behaviour is obviously due to the larva having 

 been previously attacked by a female, but whetlier it was attri- 

 butable to chemotropism, which is most probable, has not been 

 established. Tlie predominant odour was that of carrion. 



Females which have been ovipositing for some time and are in 

 a filthy condition from the carrion and putrefactive juices, will 

 endeavour to insert their ovipositor into any object which they 

 happen to feel moving, with the result that frequently two or 

 more can be seen together, " jabbing " their ovipositors amongst 

 each other's legs in a vain effort to get it firmly fixed. 



* The writer, as previously stated, is of tlie opinion that nianj' of the eggs are 

 damaged by this movement, particularly if located in or amongst muscles, 



Proc. Zool. Soc— 1920, No. XY. 15 



