294 



METAZOAN PHYLA 



Fleas make up another insect order. Since they are suctorial and 

 attack man, they are also transmitters of human diseases. The rat flea 

 seems to be the agent in the transmission of bubonic plague from rats to 

 man. 



320. Combating Injurious Insects.- — Injurious insects may be com- 

 bated in many ways. Mandibulate insects can be destroyed by placing 

 poisoned food where they will get it or by spraying poisons in solution on 

 the leaves or fruit of plants which they eat. This method can be applied 

 in the case of cockroaches, crickets, earwigs, ants, beetles, and lepidop- 

 terous larvae. Suctorial insects may be sprayed with oily substances 

 which close the spiracles and cause suffocation, or if they are in a room 

 where they can be reached with poisonous gases, they may be destroyed 

 by fumigation. Insect powders usually produce their effects by being 



Fig. 199. — Australian lady-beetle, Rodolia cardinalis Mulsant, and the scale insect on 

 which it feeds, Icerya purchasi Maskell. A, the adult beetle. B, the pupa. C, larvae 

 feeding on the scale insect. D, a twig of range, showing the scale lice, and larvae and adults 

 of the beetle. A to C, X 4; D, natural size. {From Marlatt. Year-book, U. S. Dept. Agr., 

 for 1896.) 



breathed into the tracheal tubes. In some cases insect pests may be 

 secured in numbers by jarring the plants and collecting them in appro- 

 priate hoppers, after which they may be killed in the manner most 

 convenient. Insects may be trapped in a number of ways and may be 

 combated by the spreading of infectious diseases among them. One most 

 important factor in insect control, however, is their destruction by birds, 

 and to this end insectivorous birds should be zealously protected. 



321. Beneficial Insects.— Both the larvae and the adults of the order 

 Odonata (o do na' ta; G., odontos, tooth), which includes the dragon flies 

 and damsel flies, are carnivorous, the adults being beneficial because of 

 their destruction of mosquitoes, which form a large part of their food. 



Some beetles are beneficial because they attack injurious insects. 

 Among these are the tiger beetles and ground beetles, which live upon the 

 ground and destroy cutworms and other noxious insects which they can 



