326 PARASITISM 



themselves, or other members of their species, have already 

 parasitised. Thus, Haviland (1922) has observed that the 

 Braconid Dacnusa areolaris Nees never attacked larvae of the 

 leaf-miner Phytomyza angelicas if they were already parasitised, 

 and in the considerable quantity of material examined two 

 parasites were never found in the same individual host. 



Salt (1934-37) has shown that the Chalcid Trichogramma 

 evanescens is able to distinguish between parasitised and 

 unparasitised hosts. Before parasitising a host (which is always 

 an insect ^gg) the female walks repeatedly over it and in course 

 of that process an odour is left on the egg-surface which can be 

 perceived by another female coming later. If this odour be 

 removed by washing the host-egg the second parasite is unable 

 to detect that the egg has already been attacked. After inserting 

 the ovipositor, however, the female becomes aware of the egg 

 being already parasitised and soon goes away without having 

 oviposited. If hosts are relatively few in number superparasi- 

 tisation occurs : this does not happen immediately, since the 

 Trichogramma female is able to withhold her eggs for a period 

 rather than lay them in parasitised hosts. As the density of the 

 parasites in relation to a fixed population is increased the more 

 intensive the superparasitisation becomes : when it is severe there 

 is either no resulting progeny, or the individuals of the latter which 

 emerge are dwarfed or imperfect. When given a choice of hosts 

 Trichogramma selects the larger of the eggs : also, when the 

 number of hosts is limited the larger ones are selected for super- 

 parasitisation. It further transpired that the females are unable 

 to distinguish between suitable and unsuitable non-parasitised 

 hosts and will select the latter for oviposition if of larger size than 

 the former. When given false hosts of various kinds such as flour 

 particles, tiny glass globules, sand particles. Lobelia seeds, etc., 

 the insect attempted to oviposit in these objects, where they were 

 of larger dimensions than the true hosts which were available 

 along with them. Whether this error of instinct, so evident under 

 experimental conditions, often prevails in a state of Nature is 

 unknown. 



Hyperparasitism. Probably comj^aratively few primary para- 



