418 M. Leuckart on the Reproduction of Bark-lice* 



most superficial examination, the part which the two kinds of 

 individuals have to play in the history of our Bark-lice is seen to 

 be distinct. The wingless females serve especially for the 

 maintenance, the winged ones, on the contrary, principally for 

 the diffusion of the species. The former are highly fertile for 

 a long time, but at the same time (probably in intimate connex- 

 ion with this property — see Leuckart, art " Zeugung,^^ in Wag- 

 ner^s ' Handworterbuch,' iv. p. 719) scarcely in a condition to 

 quit their dwelling-place. The existence of the species would 

 perhaps be endangered in many ways, if the timely appearance 

 of winged females did not furnish the means of finding new 

 dwelling-places and sources of nourishment. But with the 

 transfer of the ova the task of these winged females is fulfilled. 

 After depositing the eggs, a few days after their birth, they 

 perish. 



A very similar dimorphism also occurs, as is well known, in 

 the so-called nurses of the ordinary Plant-lice, which in the first 

 generations are likewise wingless, but in the later ones are almost 

 always furnished with wings. 



These are circumstances to which very little attention has 

 hitherto been paid. We usually speak merely of the difference 

 of two sexes, and tacitly suppose a complete agreement between 

 all the individuals of these sexes. With such a notion, it cer- 

 tainly appears highly anomalous when, in the communities of 

 the social insects, we suddenly meet, besides the unmistake- 

 able males and females, with other forms of individuals, and 

 recognize these as a peculiar, remarkable modification of these 

 sexual animals. The Plant-lice show us that a similar poly- 

 morphism also occurs elsewhere among Insects, and that espe- 

 cially the female individuals of these animals very frequently 

 differ from each other even by peculiarities of their structure, 

 according to the difference of their appointed tasks. 



A second observation refers to the relation of the Fartheno^ 

 genesis occurring in these Bark-lice (and in certain Coccina) to the 

 SO' called alternation of generations in the Aphides. 



That these two modes of reproduction are in many respects 

 allied and similar has already been pointed out by me in another 

 place {' Generationswechsel und Parthenogenese,' p. 44). Not 

 long ago, indeed, it was thought right to speak of an infinite 

 difference between the Aphis-nurses and females ; but such a 

 notion now appears erroneous. Our object is rather to test the 

 extent and value of these analogies — to ascertain especially 

 whether the constantly-reviving assertion recently supported by 

 Claus (Generationswechsel und Parthenogenese im Thierreiche, 

 1858, p. 22), that the so-called nurses of the Aphides are essen- 

 tially nothing but parthenogenetic females^ is correct. 



