PRELIMINARY NOTES ON THE ODONATA. 85 
seen and taken after sunset on many occasions. It must not be over- 
looked that the hour of dusk is greeted by rising hordes of mosquitoes 
and that one of these large dragon-flies possesses an enormous capacity 
and rapid digestive powers, coupled with unlimited agility. Doane, in 
his work on “Insects and Disease,” does not hesitate to say that dragon- 
flies “‘often devour large numbers of mosquitoes during the course of 
the day and evening.” 
What applies to the mosquito may be much more true in certain 
other cases. In searching for the carriers of Pellagra, a disease 
which is entering Minnesota, it has been suggested that the insect, 
if it be an insect, must be one that bites during the day and that 
lives along streams, and every effort is now being made to incrimi- 
nate Simulium. The sand-fly passes its larval stages in the water and 
the adults remain near the streams; they also bite during the day. 
It is thus more than probable that these gnats are commonly destroyed 
by many species of dragon-flies. Again, it passes without question that 
myriads of flies, including the stable fly and others which follow cattle 
about the pastures, are captured by these same mosquito hawks. The 
writer has many times observed Aeshnids darting to and fro about, and 
even beneath, the cattle along some creek or in the adjoining lowlands. 
But a consideration of the adult is less than half the story. The 
nymphs of the Odonata are doubtlesss among the most predatory and 
voracious inhabitants of the waters. Their length of life is from one 
to four years and countless adults will emerge from a single pond or 
small stream in a season. They are quite indiscriminate in their choice 
of diet, only so it be animal, and have been known to eat most of the 
common aquatic insects and larvae and in addition to occasionally feed 
upon young fish, tadpoles, crayfish, etc. They are even notoriously 
cannibalistic. 
We may, then, well expect, as is actually stated by some authorities, 
that Odonate nymphs greatly reduce the number of larvae of various 
important Diptera, the mosquito especially. The larvae of the midges 
and “punkies” (Chironomidae) have been shown by Needham and 
others to be very abundant in the habitats of the nymphs and to form a 
large part of their food supply. In reference to Simulium, Professor H. 
Garman, in his “Preliminary Study of the Kentucky Localities in 
which Pellagra is Prevalent,’ 1912, state that “Species of Pro- 
gomphus, Gomphus, Macromia, Calopteryx, etc., were collected among 
the rocks on which Simuliwm larvae occurred.” 
Regarding the relation between fish and the nymphs, there is as 
yet no decision. The work of Professor Needham in New York 
showed that the food of the brook trout consisted mainly of Chirono- 
