OUR FEIENDS THE INSECTS — BALDUF 433 



of insect parasites, Dr. L. O. Howard, who began urging biological 

 control in 18S0 and is still engaged in his favorite field. 



THE NATURE OP PARASITIC INSECTS 



A very small number of the present population of the world is 

 aware of the true nature of the white oval bodies seen so commonly 

 on the backs of certain caterpillars. Aldrovandi, in 1G02, supposed 

 them to be eggs which, he probably thought, gave rise to many more 

 caterpillars to eat the rest of his crop. His supposition also im- 

 plies that he was also unfamiliar with the phenomenon of meta- 

 morphosis of moths and butterflies. The nature of these so-called 

 " Gggs," their source, and their ultimate end could not be appreciated 

 then, as now, until the fact of metamorphosis among the parasites, 

 and their hosts as well, is understood. Parasitic insects that attack 

 other insects have a common mode of development from the egn; to 

 the parent, or adult stage. These friends of ours usually begin life 

 as an egg which the parent places into, on, or near the host. Wasp- 

 like parasites or Hymenoptera have hollow boring instruments, or 

 ovipositors, by means of which the eggs are usually passed into the 

 very bodies of their hosts, whereas two-winged flies or Diptera de- 

 posit their eggs on the surfaces of the host, and the responsibility of 

 entering the body of the latter belongs to the young parasite or larva. 

 Certain true wasps, the Tiphiidae and Scoliidae, are parasitic upon 

 beetle grubs in the soil, and their larvae are ectoparasitic, clinging 

 to and feeding only on the outside of their hosts. The Rhipipho- 

 ridae, a family of beetles, are also ectoparasites of white grubs, 

 while the little-known minute twisted-winged parasites spend the 

 larval stage in the bodies of wasps and leaf hoppers. When the 

 parasite larvae become full-sized they do, or do not, leave the host, 

 if they happen to be endoparasitic. Wasplike parasite larvae often 

 spin silken cocoons, either in the empty shell of the host, or near 

 by outside : the " maggots " or larvae of flies retain their last skin 

 instead of shedding it as before, and live in it as a covering or pu- 

 parium. In the puparium or cocoon the larva transforms to the 

 adult stage, the process of transformation being called pupation, 

 and the insect during the transition period is referred to as being a 

 pupa or in the pupa stage. Each of the four stages — egg, larva, 

 pupa and adult — is remarkably different from each of the others, 

 for which reason this mode of development is named complete meta- 

 morphosis. On the other hand, insects like the grasshoppers and 

 true bugs have young resembling the adults in form and have only 

 three stages, lacking the pupa. None of the insect parasites of other 

 insects have this type of metamorphosis. Only the larval stage of 



