No. 637] 



PARASITISM 



157 



Fulgorids have the fore tarsi of the females misshapen 

 to form chelae or pincers, by means of which they cling 

 to their host. Such structures are elsewhere unknoA\Ti 

 among insects. The group has become highly specialized, 

 apterous in several genera, and has probably reached the 

 end-stage in its evolution. Like all creatures which have 

 attained this condition, it shows no further adaptiveness 

 in habits. This is a clear-cut case of correlation between 

 habits and taxonomic affinities. 



Versatile groups naturally include large numbers of 

 genera and species with varied habits which enable them 

 to grasp every opportunity to earn (or, in the case of 

 parasites, to steal) a livelihood. Numerous species and 

 varied habits, are as inseparable as form and function. 

 The former binary involves an added series of factors, 

 since any group of insect parasites comes into keen com- 

 petition wdth the members of other groups as it reaches 

 out for new hosts. Some have spread widely among 

 hosts of very similar types, restrained by some insuper- 

 able obstacle, probably physiological in nature, from at- 

 taching themselves to strange insects. They show a cor- 

 relation between habits and structure. Others have broken 

 their fetters more quickly and completely, and adapta- 

 tions in habit have far outstripped structural modifica- 

 tions, resulting in natural taxonomic groups which show 

 only imperfectly such correlation. 



The climax in this direction is reached by certain 

 groups which have cast aside parasitism entirely and 

 become phytophagous. This has occurred independently 

 in several families of Chalcis-flies, a group in which the 

 struggle for existence must be very severe. One of these 

 aberrant series, Megastigmus and its allies (Crosby, '13) 

 feed within the seeds of plants, mainly those of various 

 trees, upon a rich protein diet, probably similar to that 

 of their entomophagous forebears. Another, Isosoma 

 and its allies (Howard, '91 and '96; Phillips and Emery, 

 '19) occur in far less delectable vegetable tissue, such 

 as the culms of grasses in which they sometimes cause 



