BY R. J. TILLYAHD. 267 



seek the protection of the river-bottoms by burrowing or burying 

 themselves under trash and rubbish. Hence might arise a 

 gradual reduction of the joints of the antennae, the slender 

 seventh joint first becoming obsolete, then the sixth and fifth, 

 and finally the already small and weak fourth joint, which we 

 now find in a rudimentary state in the present Gompldnca. Or 

 the continual burrowing miglit have so weakened these organs as 

 to cause the total disappearance, at the same time, of the three 

 top-joints together; the antenna naturally giving way at the 

 weak small joint, and conserving its remainder by the strengthen- 

 ing and broadening of the large third joint which we find in so 

 many Gomphiiice. Looked at from this point of view, the I'eta- 

 lurine nymphs are objects of great interest and of much specula- 

 tion about the past. While we can never be certain, yet these 

 ancient types point out the way by which natural selection has 

 worked its changes in past ages, and lend support to theories 

 which must command our attention and interest. 



In conclusion, Petalura is probably the oldest type of nymph 

 yet discovered. Tachopteryx apparently represents a very slight 

 departure from the older form, the change being best seen in the 

 shortening and thickening of the antennae, which represents a 

 tendency towards the Gomj^hiyice; and a slightly more developed 

 labium, which might also be regarded in that light, or possibly as 

 a tendency towards the jEschriince. 



EXPLANATION OF PLATE XXIV. 



Petalura gigantea. 

 Fig.L — Sketch showing method of attachment between male and female 



preparatory to coition. 

 Fig.2.— Ova (enlarged). 

 Fig.3.— Exuviae of male nymph (enlarged). 

 Fig.4. — Labium of ,, ,, 



Fig.5. — Antenna of ,, ,, 



Fig.6.— End of abdomen of male nymph; s, superior appendage; i, inferior: 



(enlarged). 

 Fig.7. — End of abdomen of female nymph; a, anus; fc, ovipositor: (enlarged^ 



seen sideways). . "" ' 



Figs.8-9. — Protarsus of female nymph (enlarged). yi^ ' "> / 



