stough: the hackberry psylla. 123 



acters, habits, life history, etc., listing thirteen -species. Un- 

 doubtedly the most important work published so far is that by 

 the same writer entitled "Notes on North American Psyllidae" 

 (1884). This paper lists nineteen North American species, 

 distributed among the following genera: Livia, Aphalara, 

 Psylla, Pachypsylla, Trioza, Calophyla, Rhinopsylla, and Cera- 

 topsylla, the descriptions of three genera and two species being 

 new. The most important part of this work, from the stand- 

 point of the present paper, is that dealing with the new genus 

 Pachypsylla. This genus is peculiar to this continent, having 

 no equivalent in Europe, so Riley thinks, and to it belong the 

 hackberry psyllids. Only three species of these are placed in 

 his key. Riley also discusses "hackberry galls," in the Cana- 

 dian Entomologist (1883) . 



An important contribution to the biology of the subject, es- 

 pecially of the gall-making phase, is an article on psyllid gall- 

 making in the fifth report of the United States Entomological 

 Commission, written by A. S. Packard (1887). This gives 

 keys to three species ; descriptions of pupa ; key to, and descrip- 

 tions of, the galls of the following species and varieties of the 

 genus Pachypsylla: venusta, gemma, asteriscus, umbilicus, 

 mamma, puhescens, globulus, cucurbita, and cucurbita var. To 

 this is added a detailed description of each gall and a con- 

 densed general life history of a hackberry psyllid. No mention 

 is made of the stimuli causing the gall-formation. Ashmead 

 (1881) describes the life histories and gall-making habits of 

 these insects, and lists nine species, adding three new ones. 



Two papers by Charles W. Mally, entitled, respectively, 

 "Hackberry Psyllidae Found at Ames, Iowa" (1893), and 

 "Psyllidse Found at Ames" (1894), give a series of observa- 

 tions on the winter habits and spring egg-laying of Pachypsylla 

 celtidis-mammse, with descriptions of the eggs, larvae, pupae, 

 and galls, and record thirty-one species for North America. 

 He describes four new species and figures the early stages and 

 wing venations. 



Where mention is made of the galls in the papers that I have 

 mentioned, only the gross structure of the gall is treated. Cook, 

 however, in "Galls and Insects Producing Them" (1903-'04), 

 has made a contribution toward the histology of the psyllid 

 gall ; he also discusses mouth parts and ovipositors in their re- 

 lation to gall-formation, but in conimon with other investiga- 



