AQUATIC INVERTEBRATE FAUNA OF WYOMING AND MONTANA. 217 



The food of the latter little species was peculiar at the time of my visit, and the 

 collections consequently give little idea of its usual function in the biological system 

 of the lake. All of more than fifty specimens examined from several of the Shoshone 

 Lake collections had fed freely, and often greedily, on the pollen grains of the pine. 

 Only a single specimen dissected contained also some fragments of another entomos- 

 tracau, among which were single antenual segments of a copepod, probably of its own 

 species. 



This fact is but an illustration of the dependence of the animals of a lake on the 

 contributions made to its stores of food by the surrounding land. As a surprising- 

 number of fishes profit largely by the terrestial insects falling into the water, so this 

 little copepod horde must live for some weeks, to a very large extent, on the pollen of 

 the surrounding forest, relaxing the pressure, for a time, on the plant and animal life 

 of the waters, which is doubtless their more usual food resource. Would the destruc- 

 tion of the forests here seriously diminish the number of Diaptomi, and thus lessen the 

 food supply of the young of the trout with which this lake has lately been stocked? 



The large leeches taken here occur throughout the Park in suitable situations, 

 and have been noticed by earlier collectors at Yellowstone Lake. At the Shoshone sta- 

 tion they were frequently seen in the clear shallow water, either swimming actively or 

 creeping along the bottom. They are carnivorous leeches, as already mentioned, 

 almost the only native enemies of the Gammarus worthy of notice in Shoshoue Lake. 



A frequent and interesting occurrence during our visit was the appearance at the 

 sunny surface of the lake of a large dark-gray caddis fly (Neuronia sp.) freshly escaped 

 from its pupal prison and flitting rapidly along with its imperfectly expanded wings, 

 just on top of the water, going with speed directly for the shore. The number of these 

 insects caseworms in the larval state was shown by the thousands of their empty 

 cylindrical cases washed ashore. Larva;, pupa 1 , and imagos were all common at the 

 time of our stay. The case of this species is composed of thin, irregular pieces of 

 vegetation .(largely fragments of leaves and epidermis of water plants), or of chitinous 

 plates of insects, eked out by filamentous alga; and other miscellaneous objects, all 

 cemented and imbedded in the tough secretion from the salivary glands of the insect 

 itself. On preparing to pupate, the larva closes the mouth of its tube by a coarse 

 latticework of hardened mucus which protects the insect within, permitting at the 

 same time the free access of water. Shoshone Lake, it need not be said, was an ideal 

 place for the breeding of these caddis flies, since it contained no common carnivorous 

 animal large enough to attack them. 



Chironomus larva? were common in these waters, and their pupae, ready to emerge, 

 appeared often in the surface net. 



The mollusks taken were limited to a few specimens of a large, dark Physa, with 

 an exceedingly thin and brittle shell, and a small, heavy Pisidium, with a few con- 

 spicuous lines of growth. These last were mostly empty shells, collected from the 

 hollows of rippled sandy bottom, where they were readily seen as one floated over in 

 a boat. An occasional dragon-fly larva (Libellulidw), large larv of Hydrophilus, 

 a very few hydrachnids, some slender annelids undetermined as yet, and very diffi- 

 cult of preservation and a considerable collection of digitate fragments of Spongilla 

 are worthy of mention. So also is the scarcity of waterbugs, limited, indeed, in our 

 collections from the open lake, to a single specimen each of Notonecta and Corisa. 



