6 Transactions of the Academy of Science of St. Louis 
Living usually in darkness has probably been a factor in the 
development of vestigal compound eyes or the loss of eyes. In 
some species seven or eight ommatidia are present on each side 
of the head. In others there are less than the above number 
or eyes may be absent. | 
On the ventral side of the first abdominal segment is a ventral 
tube which varies greatly in length in the different species. Sticky 
fluid on the end of this tube enables the insect to crawl on the 
under side of smooth surfaces. 
Tracheae are generally lacking so that oxygen is apparently 
taken through the moist body surfaces. However, species of the 
genus Sminthurus have tracheae in a vertical fold on each side of 
the neck. 
According to Folsom (1934) some two thousand species of this 
order have been named. Seventy-nine species have been listed 
from New York, one hundred thirty-two from Iowa, and fifty- 
eight from Minnesota. There is no list of the Missouri species 
that is at all comprehensive. 
The characteristic pontan of the collector in sathering is 
on hands and knees examining humus and damp. soil. Spect- 
mens are found in most any habitat where the relative humidity 
is sufficiently high. They are general feeders, but the, range 
of relative humidity in which existence is possible is narrow. 
Davis and Harris (1936) found that Pseudosinella violenta 
(Folsom) survives best at relative humidities between: 90- 100 
per cent, the optimum being about 100 percent. They will with- 
stand the most extreme cold and are very active while most in- 
sect life is hibernating. 
It is hard to understand why so many species are cosmo- 
politan. Of the one hundred thirty-two species from Iowa, fifty- 
nine are cosmopolitan in their range (Mills, 1934). This wide 
