382 ENTOMOLOGY 



lice are sometimes fatally entangled in the flowers of milkweed. We 

 might even go farther, and implicate all the factors that control milk- 

 weed; and so on indefinitely. Such speculation is not altogether profit- 

 less, if one bears in mind the fact that only the more immediate 

 influences are of any practical importance, and that the effect of one 

 factor may be increased, diminished, or neutralized by that of another. 



Every one of the insects or other animals that affect the clover 

 louse directly or indirectly, is itself the center of a little world of inter- 

 actions. Though we cannot follow all these interactions, their total 

 effect at any given time is expressed by the existing number of individ- 

 uals of each of the species involved; which measures also the success of 

 each species from its own point of view, so to speak. 



Equilibrium. It is not surprising, then, that species fluctuate in num- 

 ber of individuals. The presence or absence, or increase or decrease, 

 of one influence may affect many other factors, and disturb preexist- 

 ing relations. This is seen in the case of the rapid multiplication of 

 the gipsy moth and the San Jose scale insect, when introduced into 

 this country without their natural enemies. 



Evidently there is actually no such thing as a " balance of nature," 

 a true equilibrium; on the contrary there is continual fluctuation 

 within wider or narrower limits. The so-called equilibrium is simply 

 a condition of relatively small fluctuation. Under conditions of 

 nature, animals and plants approximate a condition of stability, or 

 fluctuation within comparatively narrow limits, to the benefit of all 

 concerned. Under artificial conditions, however, as when man grows 

 one kind of plant over a large area, the insects of the plant multiply 

 rapidly. Man is able to remedy such disturbances of the "order of 

 nature" in proportion to his knowledge of the factors concerned, espe- 

 cially of their relative importance. He has unwisely introduced the 

 English sparrow to subdue caterpillars; but has wisely imported and 

 propagated the native enemies of the fluted scale, the gipsy moth, and 

 other pests. 



II. CONDITIONS OF AQUATIC EXISTENCE 



The fundamental physiological requirements are the same for 

 aquatic as for terrestrial animals, but these conditions are often met in 

 different ways in the two groups. Though insects may broadly be 

 divided into these two groups, there are many kinds whose environment 

 is intermediate between water and land, and many forms are aquatic 

 in their immature stages and terrestrial as adults. 



