194 



BULIvETiN OF THE BUREAU OF FISHERIES. 



Mask. — ^The most noticeat)le thing about a nymph is the so-called mask, which is 

 folded back beneath the head and which may or may not cover the lower part of the 

 face (fig. 6), This mask is really the lower lip or labium, whose outer end terminates 

 in three lobes, one median and two lateral; the latter may take the form of stout claws 

 (^schnidge) or of spoon-shaped lamellae (Libellulidae). The mask is hinged near the 

 center and when not in use is folded at the hinge; the lateral lobes are turned inward 

 across the front end of the median lobe, and the whole apparatus is folded back beneath 

 the head. Figure 7 is a side view of the head of L. luduosa, showing the mask folded 

 back, while the lateral lobes at its tip cover the lower half of the face. This is the con- 

 dition found in the nymphs of all the Libellulidae. Figure 9 is a side view of the head 

 of Anax junius; here the lateral lobes do not cover the face at all, but extend straight 

 forward as stout claws beneath the chin. This condition is found in the nymphs of 

 the JSschnidse, with the exception of the genus Cordulegaster, and as an accompanying 



character the head is depressed or flattened. The 

 length of the labium varies considerably in different 

 dragonflies, but is usually longer in the i^schnidae 

 than in the Libellulidae and reaches back, when 

 folded, between the bases of the second legs. 



With the mask folded the nymph either conceals 

 itself in the mud or trash on the bottom or steals 

 up on its prey and when within striking distance 

 shoots the mask forward in front of the head and 

 grasps the victim between the lateral lobes. Figures 

 8 and 10 show the same two specimens of luduosa 

 and junius with the mask thus extended. 



The distance which they can reach, of course, 

 varies with the size of the nymph and the length 

 of the mask; some of the large Anax nymphs can 

 cover 15 to 25 mm. This, in addition to the 

 lurching forward of the body, enables them to 

 catch insects like Corixa, much more agile than 

 themselves, and even to capture small fishes. 



To assist in holding their prey, the lateral lobes of the mask are toothed along their 

 inner margins in the ^Eschnidae (fig. 11). In the Libellulidae they are armed with a 

 long, slender spine at the tip and a row or raptorial setae behind this along the outer 

 margin, varying in number in the different species. There is a crescent of similar 

 setae, also varying in number, along the body of the mask, called the mentum, on either 

 side of the median line. And there are more or less regularly arranged spines and 

 hairs along the remaining margins of all three lobes. Figures 12 to 14 illustrate some 

 differences between the species. 



MaxillcB. — Once grasped between the lobes of the mask, the prey is drawn quickly 

 to the mouth, where there are two pairs of organs ready to dispose of it. The first of 

 these, the outside pair, are the maxillae, which are very much alike in all nymphs; 

 each maxilla has two fingerlike branches or rami, the outer ventral) of which is 



Fig. 6. — Face of nymph of Erytliemis simpli- 

 cicollis, showing the large mask-like labimn 

 covering the lower part of the face up to the 

 antennae and eyes. 



