1965] 
Reiskind — Dimorphism in Spiders 
281 
Unzicker from a female. Both species were collected at the same 
locality, in the same way, within three days of one another. The two 
spiders are almost identical with the obvious exception of sexual 
structures. Hence it is strongly suggested that M. discreta is the 
synonym of M. rettenmeyeri. In addition, examination of an im- 
mature specimen of this species reveals a basic external morphology 
and color pattern that is in common with both adults. This species 
does not exhibit as much sexual dimorphism as in many other mem- 
bers of the Micariinae where the immature forms are even more 
useful. 
Often a species can be properly delimited only after some contact 
with the living form in its natural habitat. Both M. rettenmeyeri and 
J\l. discreta have a strange “longitudinal band of black hairs extend- 
ing along midline of cephalothorax from dorsal depression to base of 
constriction at posterior end” (Unzicker, 1965) (see Fig. 1). From 
observations and collections made by Dr. A. M. Chickering at the 
type locality in Panama it appears that in both sexes the band of hair 
enhances the resemblance to the ant Camponotus sericeiventris 
(Guerin) with which they share the same habitat. The crests of 
black hairs correspond to the solid longitudinal keel-like dorsal ex- 
tensions of the posterior sections of the thorax of C. sericeiventris 
(Fig. 2). The chances that two spiders in the same genus imitate 
the same ant at the same place and time are ecologically improbable. 
This is strong additional evidence supporting the synonymy of two 
species. 
To summarize, “alpha taxonomy” is presently a dynamic science 
incorporating many new techniques. Sexual dimorphism presents the 
problem of inadvertant splitting of species which can often be 
avoided by the use of careful comparisons as well as immature stages 
and field observations. Using these methods Myrmecotypus discreta 
is found to be the female form of M . rettenmeyeri. As first reviser 
I select M. rettenmeyeri as the valid name. 
I wish to thank Dr. Carl Rettenmeyer for the loan of the types 
of the spiders cited above. 
References 
Mayr, E., E. G. Linsley, and R. L. Usinger. 
1953. Methods and Principles of Systematic Zoology. McGraw-Hill, 
New York, 336 pp. 
Unzicker, J. D. 
1965. Two New Species of the Genus Myrmecotypus from Central 
America (Clubionidae : Araneae). Journ. Kansas Ent. Soc. 38 
(3) :2 5 3 -2 57. 
