48 CAUDAL GILLS OF ZYGOPTERID LARViE, 



The nuclpi of the hypoderm-cells are large prolate spheroids, 

 usually lying with their major axes parallel to the cuticle, but 

 sometimes standing up more or less at right angles to it. When 

 the hypodermis is exceptionally thin, the nuclei bulge inwards, 

 so that the inner border of the hj^podermis has a wavy outline. 

 The average size of the nucleus is about 8/x long, by 4/x wide. 

 Usually, the nuclei are all of about one size everywhere in the 

 gill. In the Lamellar Gills, however, the thinnest portions of 

 the blade, situated towards the distal end of the gill and beyond 

 the rachis, is bordered internally by hypoderm-cells of smaller 

 size, w4th smaller nuclei averaging only 5/x to 6/x long. 



Beneath the mid-rib, in gills of the Lestid type, the hypoderm- 

 cells tend to become columnar, and may attain to as much as 

 30/A in height. This is particularly noticeable in the genus 

 Synlesfps, where the alveolar meshwork is greatly reduced. 



A very typical section of cuticle and hypodermis is shown in 

 Textfig.l, from the latei-al (triquetral) gill of Calopterya: 



The Basal Pieces and Bi^eaking -joint. 



Each gill is attached to the tenth abdominal segment by means 

 of a short, cylindrical basal piece. Heymoiis(17) has shown that, 

 in the case of the median gill, the basal piece represents the 

 reduced tergite of the eleventh abdominal segment; while, in 

 the case of the two lateral gills, the two basal pieces represent 

 the two parts (right and left) of the reduced bipartite sternite 

 of the same segment. A section across the basal piece reveals a 

 large lumen formed of the undifferentiated hsemocoele, partially 

 filled by the large tracheae, a small accumulation of fat-cells 

 around them, and the small muscles by means of which the gills 

 are moved. 



Between the basal piece and the gill proper lies the hreaking- 

 joint, of which sections are shown in Plate iii , figs. 21, 22 {Neo- 

 sticta). It is along the transverse plane formed l)y this joint 

 that the gill is cast off, if seized by an enemy or otherwise 

 entangled, so that the larva desires to free itself. The ingenious 

 arrangement by means of which this can be done, without 

 sufficient loss of blood to be fatal to the larva, can be made out 



