The frontal sclerite is fused with the clypeus, forming the frontoclypeus. 

 The frontal sclerite forms an oblong posteriorly pointed plate in some 

 families. There are two black spots approximately at the sides in the 

 middle of their length; these spats (the tentorial pits) are depressions 

 at the points where the anterior arms of the tentorium are attached to the 

 cranium. * The position of the tentorial pits on the frontal sclerite is very 

 constant; in most forms the sclerite is broader anterior and posterior to 

 these pits, forming lateral notches in which the dark dots of the tentorial 

 pits are situated. The whole sclerite is uniformly broad in the Hydropsy- 

 chidae (Annulipalpia); the lateral notches are therefore absent and the 

 tentorial pits are displaced inward from the margins of the sclerite. 

 15 The clypeus is a narrow transversely oblong plate, the posterior margin 

 of which is fused with the frontal sclerite at the epistomal suture which 

 corresponds approximately to the line connecting the base of the mandibles. 

 The labrum is connected to the anterior margin of the clypeus by the 

 flexible, distinct and narrow anteclypeus; this plate is partly sclerotized in 

 some species. The anteclypeus is bent posteriorly by the muscles of the 

 labrum, and can be retracted together with the labrum beneath the clypeus. 



The anterior margin of the frontoclypeus bears 3 primary setae on 

 each side (anterior-marginal setae):** the median, intermediate and lateral 

 seta. The length of the anterior-marginal setae varies markedly; the lateral 

 seta is often curved; it is light and bent above the surface of the ante- 

 clypeus. The anterior-, middle- and posterior primary frontal setae are 

 situated one behind the other, at the sides of the frons, along the frontal 

 sutures; the anterior frontal seta often resembles the lateral anterior 

 marginal seta; it is situated in the middle of the anterior part of the 

 sclerite; the middle frontal seta is situated near the tentorial pits, and is 

 rarely displaced anteriorly; the posterior frontal seta is usually situated 

 in the broadest part of the posterior part of the sclerite. 



The anterodorsal region of the large lateral sclerites (parietalia) bears 

 transparent protuberances on which the eyes are situated; the rudimentary 

 antennae are situated anterior to them. 



The eye consists of a small number of facets arranged in two rows; in 

 the Philopotamidae, the anterior facets are situated at a distance from the 

 posterior facets, so that the eye seems divided. The eyes are situated 

 either near the margin of the head (species of Rhyacophila) or slightly 

 removed from it; they are sometimes situated in the middle of the head or 

 near it (Limnophilidae, Sericostomatidae). 



The antennae (Figure 4) are situated in small pits surrounded by abridge 

 which is sometimes absent. They consist of one short segment which 

 projects above the margin of the pit; this segment is sometimes reduced to 

 a small soft tubercle with one or two, rarely three, sensillae and 1—3 setae 

 at the apex which are sometimes absent. The antennae are in rare cases 

 (Leptoceridae) almost as long as the mandibles, but they are usually much 

 smaller, forming a small tubercle with a barely marked sensilla and a small 

 seta. The antennae are situated anterior to the eyes, sometimes near the 

 margin of the head. 



The labrum (Figure 5) is connected with the membranous anteclypeus by 

 a fold. The labrum is usually broader than long; the anterior corners of 

 the labrum are rounded; the anterior margin of the labrum bears a small 



* Details of the tentorium are described below. 

 "* Drawings of setae are given in the systematic part. 



12 



