536 The Animal Kingdom 



body lice, fleas, and the tsetse fly all transmit human diseases. It is 

 generally believed that the houseflies may spread the bacteria of diseases 

 such as diarrhea or typhoid ; mosquitoes transmit yellow fever, malaria 

 and filariasis ; body lice transmit typhus ; flea, bubonic plague ; and the 

 tsetse fly, sleeping sickness. 



Other insects are household pests : carpet beetles and clothes 

 moths are found in homes. The omniverous cockroach, although domg 

 no great harm, is always an unwelcome guest. Small beetles (Tribo- 

 lium sp.) often destroy flour; silverfish may eat starchy materials. 

 Among the worst of all the household pests are the bedbugs. While 

 they carry no diseases, they make life unpleasant for the hopeful sleeper. 



Vast numbers of workers are engaged in the never-ending battle 

 against these legions of insect pests. Economic entomology is an im- 

 portant phase of the study of the insects. 



IMPORTANT ORDERS OF INSECTS 



The vast number of insects is divided into more than thirty differ- 

 ent orders. Many of these contain important and familiar representa- 

 tives ; others are not so important to the general student. In the fol- 

 lowing list of orders, some of the important ones are briefly discussed. 



Class Insecta. Arthropods with three pairs of legs, various types of mouth 



parts, usually with wings, excretion by Malpighian tubules. 

 Subclass Myrientomata. Wingless insects with elongate bodies, with no 



metamorphosis, no antennae. 



Order 1. Protura. Small slender forms (0.5 to 2 mm.) with chewing 

 mouth parts, first pair of legs tactile. Segments added to body at 

 each molt. Dwell in leaf mold. About 90 species. 

 Subclass Oligoentoma. Insects with no obvious metamorphosis. Antennae 



well developed, only six segments to abdomen, no definite genitalia, lack 



Malpighian tubules. 



Order 1. Collembola (Springtails). Small wingless insects, antennae 

 and legs well developed; mouth parts of chewing type. Abdomen 

 usually with a ventral jumping organ. No metamorphosis. Found 

 in moist places such as leaf mold, surface of water, and snow. 

 A few attack vegetable crops. About 2,000 species. 

 Subclass Euentoma. Includes majority of insects. Nine or ten segments 



to body, typical characteristics of the class. 



Group Apterygota. Wingless forms. Metamorphosis not conspicuous. 



Order 1. Thysamira (Silverfish, Bristletails, Firebrats). Soft-bodied 

 forms, long segmented antennae, distinct cerci, and long caudal 

 filament. Outdoor forms feed on humus; domestic forms feed on 

 starch, cause damage to books and clothing. About 600 species. 



