. The biology of dragonflies (Odonata or Paraneuroptera). Dragon-flies. 104 THE ALIMENTARY AND EXCRETORY SYSTEMS [CH. it is an undifferentiated portion of the oesophagus, with the muscular tunic slightly stronger, and allowing of greater dilatation. Posteriorly it is closed off from the gizzard by a strong sphincter. At meta- morphosis, it becomes greatly dilated with air, as described on p. 97. In those Anisoptera in which the abdomen is swollen basally, the crop remains distended with air for a long time, thus allowing the swollen contour of the abdomen to become fixed and 4. The
. The biology of dragonflies (Odonata or Paraneuroptera). Dragon-flies. 104 THE ALIMENTARY AND EXCRETORY SYSTEMS [CH. it is an undifferentiated portion of the oesophagus, with the muscular tunic slightly stronger, and allowing of greater dilatation. Posteriorly it is closed off from the gizzard by a strong sphincter. At meta- morphosis, it becomes greatly dilated with air, as described on p. 97. In those Anisoptera in which the abdomen is swollen basally, the crop remains distended with air for a long time, thus allowing the swollen contour of the abdomen to become fixed and 4. The Gizzard (figs. 46, 47). In the larva, the gizzard hes in the second abdominal segment. It is a fairly wide bottle-shaped chamber, very highly speciaHzed. If the gizzard be slit open and cleaned, there will be seen, arranged towards the posterior end of the organ, a number of raised yellowish longi- tudinal areas, carrying teeth. These are the dental folds or fields. In Dragonflies they are nearly always either four, eight or sixteen in number. The number of the folds and the structure of the teeth are points of considerable systematic importance, which have been carefully studied by Ris [I3i] and Higgins [74]. The structure of the gizzard may be comprehended by a glance at a transverse section through the region of the teeth in any highly developed type, such as Cordidephya (fig. 47 b). We see that the epithehum and cuticle remain weak and undifferentiated between the dental folds. Beneath the teeth, however, the epithelium becomes raised up into a strong ridge of tall columnar cells {ep). The cuticle is immensely thickened, and attached to the epithehum in many places by numerous ^6n7/a€ (fi). Just above the columnar epithehum the cuticle is thick, soft and transparent (being apparently newly-formed). Further away, it forms a distinct darkened layer, which passes finally into the strong dark brown external layer of the teeth {t, t'). The whole outer layer of cuticle, includi
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