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. 



Contribution from the U. S. Fisheries Biological Station, Fairport, Iowa. 



CONTENTS. 



Introduction 



Mating and egg laying. 

 The larva 



Head 



Mouth parts 



Thorax 



Abdomen 



Locomotion 



Breathing 



Feeding 



Food 



The pupa 



Pupation. 



Description of pupa. 



Page. 

 9 

 10 

 13 

 15 

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 18 

 19 

 19 

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 21 

 22 



The adult beetle 



External characters 



Antennae 



Mouth parts 



Front tarsus of male 



Food 



Respiration 



Economic relations 



Relation to vegetation 



Relation to fertilization 



Enemies of the eggs 



Enemies of the larvae 



Enemies of the pupae 



Enemies of the adult beetles. 



General summary 



Bibliography 



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INTRODUCTION. 



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 



