19 1 2] Morphology and Biology of Insect Galls 309 



Outside of this protective zone lies the chief mass of the gall, com- 

 posed of thin-walled irregularly shaped cells, as illustrated in Fig. 15. 

 Typical cells of the protective sheath are also found scattered throughout 

 this tissue. As the gall becomes older these cells increase in number. 



Summary. 



These galls are characterized by a folding and wrinkling of the leaf 

 when they occur on that organ; in this particular they resemble the 

 Phytoptocecidia. This common characteristic is due to the fact that in 

 the orders Acarina and Hemiptera the stimulation is all from one side. 

 The spherical type of the Cynipidae is produced by a stimulus equally 

 disseminated in all directions. 



The tendency to produce trichome structures from the stimulated 

 surface, so marked a characteristic of the Acarina forms, is in this group 

 practically absent; the only hairs produced are those surrounding the 

 gall apertures. 



In most species of both groups there is little differentiation of tissues, 

 so that the protective sclerenchyma zones mentioned in the genus 

 Pachypsylla and the unclassified species on Populus balsamifera L. 

 mark a distinct advance on the specialization attained by the Acarina 

 galls and an approximation to the more complex types found in the 

 orders Diptera and Hymenoptera. 



The increassd number of resin ducts in the tissues of the Coniferae 

 stimulated by species of the genus Chermes is an important feature of 

 these galls. 



Order Lepidoptera. 



The Lepidopterous producers referred to in this paper occupy the 

 following positions in Dyar's List of North American Lepidoptera, 

 United States National Museum, Washington, 1902. 

 Fam. Sesiidae. 



Memythrus tricinctus Harris. 

 Fam,. Tortricidae. 



Eucosma scudderiana Clemens. 

 Fam. Gelechiidse. 



Gnorimoschema gallcBsolidaginis Riley. 



Gnorimoschcma gallcBasterella Kellicott. 

 Fam. Tineidse. 



Stagmatophora ceanothiella Cosens. 

 The host plants of the various species are: — 



Memythrus tricinctus Harris. 

 Populus tremuloides Michx. 



