The tube of the case consists rarely only of secretion (some species of 

 Set odes and Leptocerus). However, the outer surface of the case 

 generally bears an often heavy outer layer firmly interwoven with the wall 

 of the case. 



The simplest type of case is a straight tube slightly tapering toward the 

 posterior end, with a wide posterior opening. The external layer consists 



of fine detritus, irregularly arranged on the straight 

 tube, e. g., in the American species Phrygano- 

 psyche latipennis (see p. 162) and in the 

 temporary cases built by many larvae of Integri- 

 palpia when they hatch from the eggs. The primitive, 

 straight tubes with a broad posterior opening not 

 covered with a membrane are retained in some 

 Phryganeidae and Limnophilidae (Figures 98 and 99), 

 but in these forms the simple tube is connected with 

 a highly specialized structure of the outer layer 

 which consists of spirally arranged, plant fragments. 

 The same type of case is present in Triaenodes 

 bicolor (Leptoceridae) (Figure 100) and in some 

 species of Mystacides. 



The material of the outer layer may be inorganic 

 or organic. The larva uses sand, gravel, small 

 stones, fragments of mollusk shells, calcareous 

 crusts of aquatic plants, or diatoms and shells of 

 ostracods. The vegetable material are fragments 

 of plants or stalks of plants, fine detritus or frag- 

 ments of bark or wood gnawed off by the larvae; 

 stalks of moss or algae are sometimes interwoven 

 in the case. The longitudinal or transverse position 

 of the material is diagnostic. Longitudinally 

 arranged fragments are often arranged in regular 

 spirals; plant fragments which are arranged trans- 

 versely sometimes give the case a trihedral or 

 tetrahedral form; large pieces of wood or bark 

 give it an irregular shape. 



The portable case affords mechanical protection 

 but restricts the movements of the larvae and makes 

 them little mobile. The case often shows perfect 

 adaptations to certain conditions. These adaptations are expressed in the 

 structure and shape of the case, in the quality and size of the fragments of 

 the outer layer and in the methods of preparation and arrangement of the 

 material on the tube. The larval cases imitate the environment of the 

 larvae and are examples of mimicry which are sometimes perfect. The 

 cases of larvae living among stones resemble gravel in depressions and 

 cracks of stones, larvae living on sandy bottom have flattened cases covered 

 with sand, and larvae living among plants have green cases covered with 

 fragments of the same plants. 



Species building conical, straight or slightly curved cases covered with 

 sand (Figure 101) are widely distributed in stagnant water in the calm zone 

 of lakes (Mystacides longicornis, M. nigra, Leptocerus 



FIGURES 98-100. Straight 

 tubular cases of Integripal- 

 pia with a posterior opening: 



98 — Phryganea stria- 

 ta; 99 — Grammotaulius 

 atomarius; 100-Triae- 

 nodes bicolor. 



62 



