130 COACTION: THE INTERRELATIONS OF ORGANISMS 



is found only in a few glutinous fruits and in the so-called catchfiies 

 in which the stems may be caught by animals. Finally, there is some 

 dispersion of small seeds and fruits in mud on the feet of mammals 

 and birds, but this is far less important than has been supposed 

 (Clements, 1907). 



Dissemination by animals is often of much significance in connec- 

 tion with bare areas or those in early stages of the subsere, where there 

 is good opportunity for establishment or ecesis. It is regularly inef- 

 fective when seeds are carried into unfavorable habitats or into com- 

 munities in full occupation of the soil. When this condition obtains 

 in the community to which the species concerned belong, the conse- 

 quence is much the same, since new individuals can rarely be estab- 

 lished until mature ones die out or the turn of annuation brings tem- 

 porarily an enlarged opportunity. 



Sucking Coaction. There is a large group of food coactions in 

 which the plant is injured or killed by having the sap sucked out by 

 aphids, leaf hoppers, many other Hemiptera, certain larval Diptera, 

 and so forth. The chinchbug affords a notable example of the de- 

 struction of cultivated species by this means, but it is not especially 

 detrimental to native grasses in adjacent areas. Some forms of this 

 group alternate to some extent between plant juices and the blood of 

 animals, as well as that of man. 



While anthophilous insects secure nectar by sucking, this coaction 

 differs in practically all other respects and hence is considered under 

 symbiotic relations (page 141). 



Cambium Feeders. This coaction is characteristic of certain in- 

 sects, especially the so-called bark beetles (Scolytidae), which are 

 particularly numerous on conifers. When they occur in epidemic 

 form, they may kill trees over a considerable area, as does the Black 

 Hills beetle (Dendroctonus ponderosae Hopk.). Other borers, usually 

 Coleoptera, also girdle trees occasionally, but a much larger number 

 drill into dead wood or heartwood with little or no effect except to 

 hasten decay (see also Brues, 1920, 1924, and 1930). 



Galls. While these are not restricted to insects and mites as co- 

 actors and flowering plants as coactees, the overwhelming number are 

 concerned with these groups. The exact nature of the stimulus is not 

 known, but apparently it is due to the injection of a secretion by the 

 insect, though the use of food by the larvae may sometimes be in- 

 volved also. The three chief groups of insects are aphids, gallflies 

 (cecidomyids), and cynipids (Hymenoptera), with the spiderlike mites 

 perhaps next in importance. All plant parts may be more or less 

 affected, but the shoot, stem, or leaf figures in the most bewildering 



