INSECT ENEMIES OP INSECTS CLAUSEN 357 



Another group of insects which presents a most unusual life his- 

 tory is the wasp family Trigonalidae. The species of this family 

 are parasitic in fly pupae and in the larvae of other wasps. In 

 most instances their effect is detrimental to vegetation, as the host 

 species themselves are parasitic on various caterpillars which feed 

 on the foliage. The female trigonalid deposits her very minute eggs 

 upon the foliage of certain plants, and several thousand may be 

 laid each day for a week or more. These eggs are eaten by the 

 caterpillars as they feed on the foliage, and hatch within the diges- 

 tive tract a very short time thereafter. The young larva then pene- 

 trates the intestinal wall of the caterpillar and enters the general 

 body cavit3^ Here it searches about in an effort to find the larvae 

 of some other parasitic wasp or fly. If successful in this search, it 

 then enters the body of this larva in turn and eventually develops 

 to maturity. In this instance the chance of a successful outcome 

 is exceedingly remote, and consequently a very high potential rate 

 of reproduction is necessary to maintain the species. The first great 

 loss is suffered by the eggs themselves, as only a very small propor- 

 tion of them will be eaten by caterpillars, and those which are not 

 eaten never hatch. The eggs which are fortunate enough to be eaten, 

 however, and which hatch within the caterpillar, are still confronted 

 with the possibility that the caterpillar does not contain another 

 parasite larva. 



Where these insects occur as parasites of the nest-building wasps 

 their life history appears to be even more complex. These wasps, 

 of the genus Vespa and related forms, feed upon the body fluids of 

 caterpillars and then return to the nest and feed their larvae with 

 this material. With this host, the course of events in the life cycle 

 of the trigonalid is as follows: The eggs are laid on foliage; they 

 are eaten by the caterpillar and hatch in the digestive tract; the 

 caterpillar is Idlled by a female Vespa and its body fluids, which 

 contain the minute parasite larva, are consumed. The wasp then 

 flies back to the nest and feeds this material to its own larvae, and 

 thus the trigonalid finally reaches its ultimate host and eventually 

 attains maturity. 



It is quite clear, in view of the hazards which the trigonalid ex- 

 periences before reaching its host, that a very great loss in numbers 

 must take place. The production of 10,000 or more eggs by each 

 female is very evidently a provision to compensate for this great 

 chance of loss. Of this total, it is necessary for only 2 to attain ma- 

 turity to maintain the species. That the egg production is not ex- 

 cessive in view of the mode of life is evidenced by the fact that this 

 is one of the rarest of all parasite groups and few entomologists 

 have ever seen a living specimen. 



