288 PRINCIPLED OF ANIMAL BIOLOGY 



temperature or increatse of oxygen or starvation, and aftei* that the 

 termites are no longer capable of .subsisting on wood. Also, the protozoa 

 are unable to survive outside the termites. 



The amount of food and frequency of taking it vary greatly in different 

 animals. A certain protozoon can swallow another protozoon ten 

 times its own bidk, digest it in two hours, and be ready for anotlier; 

 while some insect larvae may eat a hundred times their own weight daily. 

 Cold-blooded vertebrates, on the contrary, subsist on small quantities 

 eaten at long intervals. Certain birds may go without food for four or 

 five weeks, a lobster for months. Some insects do all their eating in the 

 immature stages and take no food when adult; certain butterflies and 

 May flies are examples. Male rotifers get all their food by eating done 

 a generation in advance; for they take no food after hatching, all their 

 nutrition coming from material stored in the eggs from which they hatch. 



►Structural characteristics in a few animals are determined or modified 

 by their food. In the honeybee, for example, any fertilized egg may 

 tlevelop into a (jueen bee; l)ut to attain that end the lar\^ae must be fed 

 on "royal jelly," which is predigested pollen jjrepared for and given 

 to them by the workers. Other similar lar\'ae denied this food become 

 workers. A certain predatoi\y bug acquires a yello^^' color l\v eating 

 potato beetles, and the potato beetle gets the pigment from its food 

 plant. The dependence of the (jueen bee on its food has an important 

 ecological bearing, l>ut no such significance is known for the other exam- 

 ples given. 



How serious a prol^lem food is in the ecology of a species depends on its 

 food tolerance. An animal can live only where its food is obtainable, and 

 it can be very successful onl.v if its food is rather abundant, iiut some 

 animals are omnivorous, being capable of eating a wide variety of other 

 organisms, while others are very specialized. AIan>' insect lar\'ae li^'e 

 characteristically only on certain plants, numerous aphids are limited 

 to two hosts (usually one at a time), and certain parasites are found 

 only in on(^ kind of animal in each (or some) of their stages. Such animals 

 lead a precarious existence unless their food is abundant or widespread, 

 or both. 



Maintenance of Wumbers. — There are other factors wliich enter into 

 the lives of animals that help to determine their success of their distribu- 

 tion. Among them are altitude, as in mountainous regions, which affects 

 temperature and density of the atmosphere, and pressure, as of the water 

 in dee[) s(ias or lakes oi- of the aii' on mountains. The four discussed 

 above are, however, among the most important, and they will suffice to 

 illustrate the ecological situation of animals. 



Each species of animal has a certain capacity to maintain itself, and 

 this capacity is matched against an environment made up of all the 



