ESTABLISHMENT AND DISPERSION. ' 95 



spreads at a rate of but a few hundred feet per year, and if it is to 

 become generally distributed throughout the gipsy-moth-infested 

 area within a reasonable time, natural dispersion must be assisted by 

 artificial. 



These, however, are both exceptions. In the case of Monodonto- 

 merus, and perhaps of other parasites, gregarious in their habit, it is 

 not only conceivable but probable that a single fertilized female 

 would be sufficient to establish the species in a new country, because 

 the union between the sexes is effected within the body of the host 

 in which they were reared. No matter how far a female may range 

 and no matter how widely separated the victims of her maternal 

 instincts, her progeny will rarely die without each finding its mate. 

 Species having such habits are eminently well fitted to establish 

 themselves wherever they secure foothold, even in the smallest 

 numbers, and the small colony is again justified. 



Many of the hymenopterous parasites, and very likely all of them, 

 are capable of parthenogenetic reproduction, and here again is a factor 

 which becomes of considerable importance in this connection. Some 

 few of these are thelyotokous (bearing females only) and as such are 

 eminently well fitted to establishment in a new country under other- 

 wise unsatisfactory conditions. Most are arrhenotokous (bearing 

 males only), and such are probably better fitted to establishment 

 than would be the case if the species were wholly incapable of par- 

 thenogenetic reproduction. It has been proved, for example, that 

 a single female of a strictly arrhenotokous species, may, through 

 fertilization by her own parthenogenetic ally produced offspring, 

 become the progenetrix of a race the vigor of which appears not to 

 be immediately affected by the fact that their continued multiplica- 

 tion must be considered as the closest form of inbreeding. 



Whenever opportunity has offered the ability of the various 

 species to reproduce pathenogenetically has been studied, and many 

 interesting and some peculiar facts have been discovered which, it is 

 hoped, will serve as the subject for a technical paper later on. This 

 power appears to be confined to the Hymenoptera, however, and the 

 tachinid parasites, like their hosts, are rarely or perhaps never 

 parthenogenetic. 



When continued existence of an insect in a new country is de- 

 pendent upon the mating of isolated females it is at once evident 

 that it is also dependent upon the rapidity of dispersion and upon the 

 number of individuals which are comprised in the original colony. 

 One of the most constant sources of surprise is in the rapidity with 

 which the parasites disperse. One, Monodontomerus, has undoubt- 

 edly extended its range for more than 200 miles in the course of the 

 five years which have elapsed since its liberation, and there is no 





