DRAGONFUES AND DAMSELFLIES IN PONDI^ISH CULTURE. 



199 



MOUTH PARTS OF ODONATE IMAGOS. 



Dragonfly Imagos. — ^When the nymph transforms into an imago, the hinged mask 

 disappears, and the labium or lower lip is now attached directly to the ventral surface of 

 the head. In consequence there is a much greater similarity in the mouth parts of the im- 

 agos, though there are still some differences. In general, the labium of the imago consists 

 of a basal portion, corresponding to the mentum of the nymphal mask, but destitute of 

 raptorial setae and very much shortened. From its anterior margin projects the median 

 lobe, greatly enlarged in Anax and i^^schna (fig. 29) and covered with hairs, but reduced 

 to a small triangular piece in the Libellulids (fig. 32) and more or less free from hairs. 



On either side of the mentum is a side piece known as the squame (ms) which sup- 

 ports the lateral lobe. These latter now fold together across the front of the face in all 

 the imagos. In Anax and i^schna they are enlarged into concave lamellae, strongly convex 

 on their outer borders and covered with hairs. The inner border ends distally in a sharp 

 end hook (e) which is immovable; just outside of this is a larger movable hook (mo), 

 which is rounded and palplike and covered with hairs. When the lobes are folded 



across the front of the face, these 



four hooks meet on the median 

 line, but the margins of the lobes 

 beyond them diverge rapidly. 

 In Erythemis (fig. 32) and the 

 other Libellulids the margins of 

 the lateral lobes, when folded, 

 meet each other in a long median 



Figs. 35 to 3 7 — Imago of Enallagma hageni: 35, mask: 36, maxilla: 37, man- Jijig^ both hookS haVC practically 

 dible. end hook: twZ, median lobe of labium: mo, movable hook. . ^ ^. 



entirely disappeared, and the 

 lobes are covered with hairs. The mandibles (figs. 31 and 34) and the maxillae (figs. 

 30 and 33) have changed a little in detail, but are practically the same as before. The 

 teeth of the mandibles are very strong and may be divided into two sets, the incisors 

 (in) at the tip of the mandible, which are long, curved, and sharply pointed, and the 

 molars (m) near the base, which are much shorter and armed with separate cusps or 

 points. The maxillae still retain the outer lobe or palp (p), which is curved and covered 

 with hairs, and the inner lobe, which is armed with curved and sharply pointed teeth 

 and a pad covered with long sensory hairs. 



Damselfly Imagos. — In the damselflies the general structure of the mouth parts 

 is the same as in the dragon flies. Here the median lobe of the labium (ml, fig. 35) is 

 fully as long as the lateral lobes, is divided by a deep median fissure, and is covered with 

 long hairs. The lateral lobes retain the immovable end hook (e) , which is very long and 

 slender and curved to an acuminate point, and the movable hook (mo), which is also long 

 and narrow, but is bluntly rounded and covered with hairs. These lobes are relatively 

 much narrower than those of the dragonfiies, and, when folded, only the terminal hooks 

 meet on the median line. The mandibles (fig. 36) and maxillae (fig. 37) are similar in all 

 respects to those of the dragonfiies, except that there is a sharper distinction between the 

 incisors and the molar in the mandibles, while the maxillae have long hairs on the outer 

 margin near the base. 



The gizzard in the imago is relatively much smaller and weaker in the nymph, and 

 has very little functional use. The chitin ridges or folds along its walls are still retained, 



