NORTH AMERICAN ODONATA. 195 



growths of the germinal band is unknown. No account exists of 

 the formation of the endoderm and mesoderm, or of the internal 

 organs. 



THE NYMPH. 



The term nymph is here used to denote that stage of Odonate ex- 

 istence between the egg and the transforn;ation into the imago. The 

 length of time which elapses between the hatching of the egg and 

 imagination (nymphal period), and the number of moults during 

 that period, have not apparently been recorded for any species. It 

 seems probable that the time extends over a considerable number of 

 months, at least in the larger species, and that moulting occurs many 

 times. 



The nympli of Anax JMni?<s, just hatched, is 2 mm. loug; the length of the 

 nymph skin remaining after imagination is 45 mm. A nymph of this species ob- 

 tained in August, 1888, moulted September 3d, the cast-oflF skin being 1.5 mm. 

 long; it moulted again September 20th, the length of the exnvia was about 17.5 

 mm. ; this nymph died before moulting again. According to Poletaiew (5), the 

 rudimentary wing-covers only appear in Odonate nymphs after the third or 

 fourth moult; both of these last mentioned exuvise show these covers. These 

 facts appear to support the statement just made, although, of course, captivity 

 may have retarded the rate of growth. 



The young nymph, when hatched, is about 1 mm. long in Libel- 

 lula pulchella, 2 mm. in Anax Junius. In all the groups it is flat- 

 tened from above downwards, the head is large and distinct, the three 

 thoracic segments are clearly visible, and likewise the ten abdominal 

 segments. The eyes are relatively small, and in all the groups are 

 widely separated from each other. The antennae, on the contrary, 

 are much longer than in the imago, being as long as the head in the 

 young nymph of 3Iesothenm slmpllcicollis ; thev consist of three 

 joints, of which the terminal one is much the longest, except in the 

 legion Calopteryx. No ocelli are visible. Both mandibles and first 

 maxillae are present, somewhat similar to those of the imago. The 

 labium (second maxillae) has the form which is so characteristic of 

 the Odonate nymphs (see PI. II, fig. 8), and is known as the mmk.'^ 

 It is proportionally very much longer than in the imago. It con- 

 sists of a basal piece (subraentum), attached to the head, and which 

 at its distal end bears a second piece {m and ml, fig. 8), correspond- 

 ing to the mentuni and median lobe of the imago together. The 



*■ So-called from the circumstance that in many genera, e. g. LibeUuIa. the lat- 

 eral lobes are so large as to cover (mask) a great part of the face, when folded 

 in rest. 



TKANS. AM. ENT. SOC. XX. AUGUST, 1893. 



