162 
$ .Terminal abdominal segment strongly tapering and notched 
at the apex. 
Ega, Upper Amazons. 
The very great and striking difference in the accessory genital 
organs between these two closely allied species {Colohothea securi- 
fera and C. sejuncta) merits a few words of especial mention. 
When I was separating my specimens of Colohothea into species, 
I placed together all the individuals belonging to these Wo as 
one and the same, and could not find anything in their form or 
markings to warrant their being treated as anything more than 
mere local varieties, even after I had given them a second exa- 
mination. A species has so often proved to exist under distinct 
local forms on the Upper and Lower Amazons, that I concluded 
this was simply another example of the rule. When I came, 
however, to separate the sexes previous to describing the species, 
I discovered the remarkable difference of structure described 
above, and then noticed the two or three other small points of 
difference in the general shape and tips of the elytra which I 
have noted in the descriptions. A pair of elongated horny pro- 
cesses, which I suppose to be the sheath of the penis, project 
from between the terminal abdominal segments in two out of 
the three males I possess ; in the third they appear to be with- 
drawn into the abdomen. It is a remarkable circumstance, that 
in many families of Insects which have accessory sexual parts 
easy of examination, it is found that these differ very con- 
siderably in structure in closely allied species. It has been 
remarked that they offer some of the best eharacters to distin- 
guish species, and they have been made use of to separate species 
which scarcely offered any other distinguishable characters. 
Mr. Baly has also discovered that the horny penis concealed in 
the male abdomen of Phytophagous Coleoptera differs in form 
in closely allied species ; and he has shown me a long series of 
specimens mounted for examination under the microscope, be- 
longing chiefiy to the genera Chrysomela and Eumolpus, which 
offer a most instructive study, since by their means some forms 
before considered as varieties turn out to be distinct species. 
This class of facts seems to me of great significance, as throwing 
light on the segregation of varieties and their passage into true 
species. For if we admit that the only sound difference between 
allied varieties and allied species is that the former intermarry, 
and the latter do not, then the abrupt and great diversities of 
structure in those organs most directly involved in the matter 
must be considered as affording an explanation why many varie- 
ties do not intercross with the parent stock, and therefore re- 
main as independent forms or species. The difference in the 
accessory male organs of our two allied species or local forms of 
