NO.1310. NORTH AMERICAN THYSANOPTERA— HINDS. HI 



any time; in .still others, while the females are very abundant, males 

 are unknown. The explanation for the relative scarcity or absence of 

 males is found in the second method of g-eneration. 



Unisexual reproduction. — Parthenogenesis is the usual mode of 

 reproduction in at least ten species, all Terebrantia, and probably 

 occurs very frequently in many others, though positive statements 

 can not l)e made upon this point until more extensive collecting has 

 been done and life histories have become better known." 



It seems that parthenogenesis must take place to some extent in 

 those species in which the males are comparatively rare or arc active 

 for only a short season. However, no su ^h thing as a regular alterna- 

 tion of generations, as in Aphidc^, is yet known to exist among Thy- 

 sauoptera. In his studies of Parthenotii -ips dracmnx Jordan found 

 that the normal method of reproductior in warm greenhouses was 

 unisexual, Avhile on plants standing in a ool room an abundance of 

 males was developed, and this condition lasted in the cool room 

 throughout the winter season. The males of Aptinothrijys rufiis have 

 been found only at haying time, and then only very rarely. 

 i 



DISSEMINATION. 



It has already been noted that in most species there appear for some 

 part of the season, in some generation or in one sex, individuals bear- 

 ing full}^ developed wings, and we can not doubt that the wings play 

 a large part in the distribution of the species. Certain it is that the 

 power of flight is greater than would seem possible with such delicate 

 Hvings as these insects possess. After harvest or toward autumn some 

 species fly in large numbers, and in some instances have caused con- 

 siderable annoyance by entering houses for hibernation. Winds may 

 easily carry them for considerable distances, and when so scattered it 

 is evident that their power of parthenogenetic reproduction is of great 

 lassistance in the establishment of the species in a new localit3\ Spe- 

 bies living under the bark of trees growing upon the banks of streams 

 are probably often carried for long distances on wood floating in the 

 water, as some species which have been observed are found to endure 

 ^ large degree of moisture and even submersion for some time without 

 ^i^ji^iy^ aw<l moist, decaying wood is their normal food. Species living 

 ppou cultivated plants, as in greenhouses, have doubtless been dissemi- 

 Qated in commercial ways. Strange as it may seem, a species which 

 is entirely wingless {Aptinothrips rufus Gmelin) is one of the most 

 svidely distributed. It is hard to believe that this species can have 

 ittained its present distribution in both Europe and America through 

 :he slow method of crawling. 



« Males of the following species included in this paper are rare or unknown: Par- 

 henothrips dracamw, Heliothrips ha-morrhoidalis, Aptmothrlps rnfm, Anaphothripn stria- 

 us, Thrips tahacl. Some others are too little known to be placed here. 



