LIFE HISTORY OF THE SCAVENGER WATER BEETLE, 
HYDROUS (HYDROPHILUS) TRIANGULARIS, AND ITS 
ECONOMIC RELATION TO FISH BREEDING. 
* 
By CHARLES BRANCH WILSON, Ph. D., 
State Normal School, Westfield, Mass. 
eM 1 
Contribution from the U. S. Fisheries Biological Station, Fairport, Iowa. 
J- 
CONTENTS. 
Page. 
26 
27 
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29 
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30 
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32 
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35 
36 
INTRODUCTION. 
Page. 
Introduction 9 
Mating and egg laying 10 
The larva 13 
Head . 15 
The adult beetle 
External characters 
Antennse 
Mouth parts 
Mouth parts 16 
Thorax 17 
Abdomen 17 
Locomotion 18 
Breathing 18 
Feeding 19 
Food ... 19 
Front tarsus of male 
Food 
Respiration 
Economic relations 
Relation to vegetation 
Relation to fertilization 
Enemies of the eggs 
Thfi pupa _ 21 
Enemies of the larva; 
Pupation 21 
Description of pupa 22 
Enemies of the pup® 
Enemies of the adult beetles 
General summary 
Bibliography 
In recent revisions of the scavenger water beetles Say’s species triangularis has 
been referred to the genus Hydrous instead of Hydrophilus, and the latter genus 
has been considerably restricted. This revision is fully accepted, but to facilitate 
recognition both names have been included in the title of the present paper. 
The European species, Hydrous piceus, has been described and figured in detail 
by many eminent entomologists, but our American species have thus far received 
only scant attention. This is the more to be regretted because they differ in many 
important particulars from their European relatives. 
The beetle of the present investigation always forms, or is likely at any moment 
to become, one of the important factors in the life of every fishpond. Hence, an 
exact knowledge of its habits and life history is essential if we are to deal with it 
intelligently. During the summer of 1918 an unusual opportunity for obtaining 
the life history of this beetle was presented at the United States Fisheries Biological 
