806 THE AMERICAN NATURALIST. [VoL. XXXV. 
Leptothorax is a guest or social parasite resembling Anergates, 
though still retaining intact its own household and its domestic 
instincts. Z. emersont may therefore be said to combine in 
itself the instincts of ants belonging to several categories of 
mixed and compound nests. 
The range of habits clearly indicated in this brief survey of 
our very fragmentary knowledge of Leptothorax species is 
still further enlarged if we include the genera Tomognathus 
and Formicoxenus, both of which are closely allied to Lepto- 
thorax. In fact, Tomognathus is indistinguishable from Lep- 
tothorax in the male sex and larval stages (Adlerz, '96). That 
the three myrmicine genera under consideration must have had 
a common origin is evident from their morphology. Neverthe- 
less the habits of the various species are so diverse as to repre- 
sent all the forms of social symbiosis except colacobiosis of the 
extreme type found in Anergates. It is evident, furthermore, 
that the ants of these genera must have originally possessed 
certain traits which made it especially easy for them to enter 
into symbiotic relations with other species of Formicide. I 
believe that we may still recognize in many of the species of 
Leptothorax several of these traits, such as the following: 
1. The genus has a very wide geographical distribution, a 
prerequisite to the establishment of such numerous and varied 
relations with other ants. 
2. The species are all of small size. This must undoubtedly 
facilitate their association with other ants. 
3. The colonies consist of a relatively small number of indi- 
viduals. This, too, must greatly facilitate life as guests oF 
parasites in the nests of other ants. 
4. Most of the species are rather timid, or at any rate not 
belligerent. They are, therefore, of a more adaptable tempera- 
ment than many other ants even of the same size (eg. Tetra- 
morium cespitum). Forel (74, pp. 339, 340) has shown that 
L. tubero-affinis will rear pups of L. nylanderi and even of 
Tetramorium cespitum and live on good terms with the 
imagines when they hatch. 
5. There is no very sharp differentiation in habits b 
the queens and workers of Leptothorax. This, too, should 
etween 
