Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 97 



typical predators. Parasites are smaller than their host, feed upon its substance 

 persistently, and chronically weaken or gradually kill it. A field mouse can 

 supply blood to a moderate -population of lice without great injury. But an 

 excessively large population results in great competition among the lice and 

 the death of the mouse from loss of blood. Like tax collectors after more in- 

 come the lice must then find another mouse. 



Development of Parasitic Living. In the early stages of parasitism the in- 

 cipient parasites visit their hosts only for meals. Blood-sucking leeches clamp 

 their suckers to the flesh, insert their jaws, suck blood until they are satiated, 

 and then drop off into the water. Such a meal supplies a leech with food for 

 several weeks. The blood-sucking mosquitoes, always female, spend even less 

 time on their hosts and simply take a firm stand on the skin while they suck 

 up the blood (Chap. 30). In certain species mosquitoes do not lay their 

 eggs until after they have had a blood meal. In laboratories where they are 

 reared they are allowed to bite a human victim whenever eggs are needed 

 for experiment. Such mosquitoes have taken a long step into parasitism 



-is. ^ LARVA 



^.j ^/ (CHieeiR) 



EGG 



o 



ADULT 



Fig. 6.6. Ectoparasites; examples of parasitic life on the external surface of the 

 body: fleas, lice, chiggers. Left, the common rat flea, Nosopsyllus fasciatus (after 

 Bishopp) : upper, the nonparasitic larva and pupa that live near the host, not upon 

 it; lower, the blood-sucking parasitic adult (female) that stays much of the time 

 feeding on its host, slips easily between the hairs, has great ability to spring on and 

 off its host, and is able to adopt a human one temporarily. Center, common chig- 

 ger, or jigger mite, harvest mite, Entrobicula alfredugesi. The exceedingly minute 

 six-legged parasitic larva that bores into the skin, liquefies the local tissue and 

 sucks up the fluid. After feeding the mite is no longer parasitic but drops to the 

 ground and develops the free living eight-legged stage, Chiggers are distributed 

 from New York to Minnesota and are pests in the southern states attacking all 

 land vertebrates including man. Right, human head louse, Pediculus humaniis. var. 

 capitis. Adult showing the claw and thumb that lock around the hairs. Lice are 

 highly adapted for clinging and blood sucking and do both throughout their life 

 history. {Left, courtesy, Matheson: Medical Entomology. Ithaca, N.Y., Comstock 

 Publishing Co., 1950. Center, courtesy, Stiles: Individual and Community Health. 

 New York, The Blakiston Company, 1953. Right, courtesy, Herrick: Household 

 Insects. New York, The Macmillan Co., 1916.) 



