530 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



Other morphological structures have been greatly neglected. The internal 

 morphology of a few of the larger species has been studied, but more careful 

 studies of the internal morphology of many of the smaller species are needed. 

 I feel safe in saying that we do not have sufficient knowledge of the morphology 

 of enough representatives of the various families, tribes, and subtribes to gen- 

 eralize about the phylogeny of the group as a whole. 



The study of the physiology of the Homoptera has been woefully neglected. 

 We have perhaps a Ijeginning of comprehension of their methods of feeding, 

 and a little knowledge about their digestion, especially in some of the vectors 

 of plant diseases. A startling discovery by Ossiannilsson that all these insects, 

 and not the cicadas alone, are singing insects, is perhaps one of the most inter- 

 esting developments. Except some minor contributions on the secretion of wax, 

 honeydew, the formation of froth in the froghoppers or spittlebugs, there is little 

 of real importance in the study of the physiology of these insects. The study of 

 many other physiological aspects awaits better techniques than any now available, 

 especially for the investigation of the smaller forms. 



In the field of ecology most of the contributions have been on the food plants 

 of the various species. One would gather the impression that these insects were 

 almost exclusively confined to a single host plant or to a very limited order of 

 host plants, and that only a few species are rather general feeders. My own 

 impression from limited study would lead me to believe that the exact opposite 

 may be true and that the limiting factor is perhaps the sum total of all tlie 

 physical and biological features of the environment. Thus a species, if it finds 

 other favorable physical and biological factors, may transfer its attention from 

 one host plant to another belonging perliaps to an entirely different group of 

 plants. Now such an assertion as this is exceedingly difficult to prove because, 

 in the first place, we cannot at present be even reasonably sure what the physi- 

 cal or biological factors in the environment are or what is the insect's ability 

 to adapt itself to their extreme ranges. Neither can we be sure that we know 

 the most important physical and biological factors in the environment of these 

 insects. We assume that temperature, humidity, and food plants rate very high, 

 but we have very little evidence of their importance. 



As illustrations of these two points I have only to report three limited ol> 

 servations. What is apparently the same species of small planthopper was 

 described originally from Spartina grass growing on the high dunes of Long 

 Island, and has been taken also on a species of TJniola growing on the high dunes 

 along the North Carolina coast. Here we have, apparently, two different regions 

 with approximately the same physical factors harboring the same species of 

 insect. In Northern Michigan, however, I observed another larger species of 

 planthopper living in tlie sheltered beach pools on rushes, whereas this plant- 

 hopper was not found along the shores of tlie lake where the rushes were sub- 

 ject to high winds. Every student of this order who lias collected extensively 

 in the field has had this experience. Two nitches, wliich are as far as can be 

 judged identical in the more important biological and physical factors, are 

 vastly different in regard to the total population of Homoptera; for the one will 

 yield a large number of specimens whereas the other seems to have none. What 

 then are the factors that make such conspicuous differences? Whether any of 

 these observations will stand the test of carefully planned experiments with 



