PLANTS IN PONDFISH CULTURE. 7 



Previous to the identification of tliis chironomid in the fishponds 

 the inventory of the phmts had disclosed the interesting association 

 of the larval stage of this insect with the alga, Mougeotia genurfiexa.. 

 The alga was exceedingly abundant and produced a most remark- 

 able and beautiful formation. This formation, or algal mat, as it 

 may be called, was composed of delicate, light-green filaments of 

 gossamer-like threads which were interwoven intricately in the form 

 of loose-meshed rolls suspended near the surface of the water. The 

 author's attention was attracted by a large number of elliptical ly- 

 shaped bags, or cases, lightly held in the meshes of the algal mat. 

 These proved to be the larval cases of the chironomid in question. 

 While foraging for food the larvae move in and out of their cases, 

 retaining their hold, meanwhile, by their caudal hooks. In feeding 

 they draw toward them filaments from the loose meshes of Mou- 

 geotia, or they forage directly from the outer walls of their cases, 

 to which this alga is applied in the construction of the case, renew- 

 ing the filaments as fast as they become depleted. The larvae con- 

 tinue to feed upon this alga as long as the supply lasts, which is 

 generally only until the middle of July, for by that time this species 

 of alga, which flourishes during the early part of the season, has 

 passed the stage when floating mats are readily formed. There- 

 after it becomes scattered through the partial disintegration or break- 

 ing up of the filaments. It is at this time that the characteristic 

 " knees " or swellings in the cells occur which give to this form of 

 Mougeotia its specific name, geriufexa. It is while engaged in 

 these feeding operations that the larvae fall prey to the bass. 



Several hundred larvae of OrtJioclculms nivoriundus were examined 

 to determine to what extent other algae contributed to their food 

 supply. The determination is a simple matter. The larvae are 

 lightly crushed under the cover glass by a slow, forward movement 

 of the thumb, a slight pressure being sufficient to push out the con- 

 tents of the digestive tube. The material is teased out under the 

 microscope, and the problem becomes one of identification of the 

 algae. Almost without exception larvae taken from the mats of 

 Mougeotia showed a forage value of 100 per cent of this alga. From 

 larvae taken at random in pond 3D, from habitats in which other 

 algae were the chief constituents and Mougeotia appeared only as a 

 small factor in the heterogeneous assortment, this alga again formed 

 the bulk of the food. Specimens of 0. nivorimidus were hatched in 

 a watch glass to which had been added a variety of algae as well as 

 Mougeotia, and it is interesting to note that the young selected the 

 latter invariably. It is not surprising, for Mougeotia is one of the 

 most delicate of the filamentous forms and possesses a brittleness 

 which apparently recommends it to the larvae, for when observed in 

 their feeding operations they were seen to snap off filaments in one- 

 cell and two-cell lengths with gi-eat rapidity. This preference for 

 Mougeotia was observed in the very earliest stages of larval growth, 

 and it continued to be shown to pupation. 



Later broods of Orthocladius nivorlundus^ of which there appeared 

 to be three during the summer, found their natural forage among 

 other algae which succeeded Mougeotia gemoflexa, such as M ougeotld 

 spluerocco-pa,, fSpirogyra nitida^ S. 7n.ajusciita, QEdogonium, llydro- 

 dictyon reficulatu-m^ Lyngbya, and various microscopic forms present 

 in the algal mats. 



