OF WASHINGTON. 9 



two for the crow, three for the cutworm, and three to grow," 

 so nature calculates that in Argiope, for example, so many 

 young are for the parasites, so many for mud-wasps, so many 

 for the birds, and the rest to grow. The two groups are 

 closely interrelated. For as the protectively-colored group 

 becomes more definitely protected, so the extremely fertile 

 group must become more fertile to supply the increasing 

 amount of loss coming to it. It is evident, therefore, that 

 there can be no final balance between the two groups, but 

 rather a fluctuation, due largely to the varying abundance of 

 their enemies from year to year. The success of the pro- 

 tectively-colored group must not be too pronounced, else it 

 would become abundant and more liable to capture ; but there 

 must always be a few captured to maintain the protective de- 

 vice in its present condition, else there would be reversion to 

 the more normal coloration. 



As an appendix I might add that none of our spiders 

 strongly resembling ants was found in any of the mud-wasp 

 nests, the nearest approach to them being Salticus albocinctus, 

 two specimens of which were taken by a wasp at Falls Church. 

 Synemosyna formica, our most common ant-like spider, the 

 species of Peckhamia, of Tutelina, of Castaneira, and of 

 Micaria were not found in the cells. All are found running 

 exposed in the day-time and resemble ants. Mud-daubers do 

 not take ants, so that this form of mimicry is very valuable to 

 these spiders ; however, they become subject to the attacks of 

 the enemies of ants, which include various birds. This also 

 goes to show that these ant-like spiders are not trying to es- 

 cape from birds, but from their more serious enemies, the 

 wasps. 



In the discussion which followed, Mr. Webster described 

 a fly, a Trypeta, reared from galls in ragweed, which had 

 come under his observation, and which closely mimicked a 

 spider, the wings being so colored as to represent the legs. 

 Not only in color, but also in its movements was the similar- 

 ity very marked. He argued from this that the spider which 



