STUDIES OF VARIATION IN INSECTS 313 



8, 4-9; seg. 9, 7-9. The length numbers refer to units of the 

 micrometer scale, not reduced to millimeter fractions. 



It is surely obvious, as has been mentioned before in connec- 

 tion with the account of the variations in the number of tibial 

 spines of Melanoplus femur-rubrum (p. 34) that systematic 

 students of zoology must take into account the conditions of 

 variation exhibited by the characters they choose for use in 

 specific diagnoses. Students of variation may never supply 

 biology with " a precise criterion of species " but they can very 

 promptly supply the systematist with a precise criterion of the 

 stable value of any specific character. 



Variation (Absent) in Number of Antennal Segments in 

 Eleodes sp. and Vespa sp. In a lot of 123 adult individuals 

 of Eleodes sp., a darkling ground beetle, collected at various 

 times and places near Stanford University, no variation in the 

 number of segments composing the antennae was found. The 

 normal number is eleven. The insect has a complete meta- 

 morphosis. 



In a lot of 55 adult individuals of Vespa sp., yellow-jacket, 

 collected on one day in one place (feeding on refuse), on the 

 Stanford University campus, no variation in the number of seg- 

 ments composing the antennas was found. The normal num- 

 ber of antennal segments is thirteen. The insect has a com- 

 plete metamorphosis. The same individuals show considerable 

 variation in pattern of abdominal markings (see p. 284). 



Variation in Number of Long Tactile Hairs on the Meta- 

 thorax of Lipeurus celer and L. varius (Biting Bird Lice). 

 The biting bird lice, or Mallophaga, are external wingless 

 parasites on birds and mammals, living their whole life on the 

 body of their host or in some cases on the bodies of two or more 

 hosts, migrating from parent to young in nesting time, from one 

 sex to another in mating time, or from one companion to another 

 in the case of crowding, gregarious species. Their migrations 

 however, are practically limited to passing directly from one 

 host to another when the hosts are in actual contact and the 

 majority of parasitic individuals undoubtedly spend the whole 

 life from egg to death on one host. Thus there exists a pro- 

 nounced isolation of groups of individuals on individual hosts 



