83 



RECENT INVESTIGATIONS IN INSECT PARASITISM. 



By Otto H. Swezey 



(A paper read before the Ag'ricultnral Seminar, College of 

 Hawaii, February 15, 1912.) 



Insects as parasites have been known since the remotest times 

 of human history. That insects, themselves, were parasitized by 

 other insects was demonstrated in the later centuries, when close 

 attention began to be given to biological studies. This gave rise 

 to the familiar rhyme : "The little fleas which do us tease, have 

 other fleas to bite 'em : and these in turn have littler ones, and so 

 ad infinitum." Though not true of the flea, this idea is exempli- 

 fied in many groups of insects that have been exhaustively studied 

 in quite recent years. 



At the first, let us ask. "What is meant bv insect parasitism?" 

 Parasitism has been defined as an association of two different 

 animals, one, the parasite, living at the expense of the other, the 

 host. Manv of the parasites with which we are familiar do not 

 kill the host outri^-ht. but live along- with the latter continually, 

 often for long- periods of time, or acting only as an annoyance 

 for shorter periods of time : as, for example, lice and fleas on our 

 domestic animals. 



On the other hand, when one insect parasitizes another insect, 

 it usuallv results in the death of the host ; as, for example, the 

 Tachinid fly, which lavs its eggs upon army worms and other 

 caterpillars. When the egg hatches, the voung maggot pene- 

 trates the body of the caterpillar, there living and growing by 

 feeding on the juices and fat of the host, not attacking the vital 

 organs till the young parasite nearlv reaches its full growth. It 

 finallv does destrov sufficient of the vital organs as to result in 

 the death of the caterpillar, which has all along been supplying 

 the nutriment for its growth : then leaving the dead corpse of 

 its host, enters the groimd to finish its transformation to the 

 adult stage. 



A predaceous insect also causes the death of the insect on 

 which it feeds, but it is a more sudden process ; as, for example, 

 when a ladvbird beetle eats a plant-louse, it is a momentary af- 

 fair. This illustrates the difference between insects that are para- 

 sites and those that are predators. However, the line between 

 the two classes cannot be very accuratelv drawn. To illustrate : 

 When the little Braconid fly stings and paralyzes a palm leaf- 

 roller caterpillar, lavs its eggs on the surface of the caterpillar, 

 and these hatch and feed externallv on the paralyzed caterpillar, 

 in two or three days completing their growth, then spinning their 

 silken cocoons on the leaf nearbv. from which the adult insects 

 emerge in about a week, we call this insect a parasite. But when 



