90 BR. J. ENT. NAT. HIST., 7: 1994 



Fig. 2. Posterior adhesion point in late fourth instar larva of Chironomus annularius. a: adhesion 

 point between larval and pupal cuticles; b: larval cuticle; c: pupal cuticle; d: posterior margin 

 of abdominal segment V. Scale line 0.1 mm. 



there are no muscle attachments to the cuticle beneath the anterior and posterior 

 marks. 



In late fourth-instar larvae, the nearly fully formed pupal cuticle remains attached 

 to the otherwise apolysed larval cuticle at all the points that remain smooth upon 

 ecdysis (Fig. 2). At this stage the larva is still feeding, and its movements are still 

 larval: it crawls using its anterior and posterior parapods, and remains capable of 

 curling into a spiral when disturbed. Shortly before ecdysis, the lateral adhesion points 

 are seen to be separated from the larval cuticle. The separation of the cuticles at the 

 anterior and posterior marks occurs later, upon ecdysis. 



Discussion 



The late fourth-instar chironomid larva is a complex animal. In Chironomus the 

 pupal head (and the developing adult head within it) forms in the anterior thorax 

 of the larva (Miall & Hammond, 1900). (In many other chironomids the pupal head 

 develops partly in the posterior part of the larval head and partly in the anterior 

 thorax.) Apolysis of the larval cuticle is progressive, beginning at the developing wing 

 and leg sheaths, subsequently spreading around the thorax and along the abdomen. 

 Even when the pupa is nearly fully apolysed and adult features can be seen within 

 it, the animal is still behaving as a larva and continues to feed. Final preparation 

 for ecdysis is very rapid. Extensions into the larval head and parapods are withdrawn. 

 The pupal abdomen becomes dorsoventrally flattened, presumably by the contraction 

 of the lateral dorsoventral muscle bands. At the same time the pupal cuticle comes 

 away from the larval cuticle at the lateral adhesion points. Movements now are the 

 dorsoventral undulations characteristic of the pupa, which serve to drive the pupa 

 forwards in the larval cuticle and out through the split dorsal suture of the larval 

 thorax. During this process the pupal and larval cuticles pull apart at the anterior 

 and posterior adhesion points. 



The adhesion points between the pupal and larval cuticles enable the larva to transmit 

 its movements to the substratum from musculature now enclosed within the pupal 

 cuticle. The time between the cessation of larval feeding and the emergence of the 

 imago is thus greatly reduced, for the adult is already nearly fully formed when the 

 pupa leaves the larval exuviae. (In Paratany tarsus grimmii Schneider imaginal eclosion 

 may take place only 25 minutes after pupal ecdysis (Kriiger, 1941).) 



