LIBELLULID^. 



[ 386 ] 



LIBER. 



depressed, oval or oblong, uniformly rounded 

 at the two ends, and covered with long cilia 

 in very numerous parallel rows, and without 

 a mouth. The species are probably Opa- 

 lince. 



PL 24. fig. 37 represents L. striata, D. 



BIBL. Ehrenberg, In/us, p. 311 ; Dujar- 

 din, In/us, p. 458 ; Stein, In/us, p. 184. 



LIBELLULID.E. A family of Insects, 

 of the order Neuroptera. 



It contains several common but beautiful 

 insects, some of which are popularly known 

 as dragon-flies and horse-stingers, although 

 they are harmless. 



The great interest connected with them 

 relates to the structure of the larvae and 

 pupae, which live in the water, and are fur- 

 nished with branchiae, either internal or ex- 

 ternal, and situated at the end of the body. 



External branchiae are seen in Agrion 

 (PL 28. fig. 17). They consist of three 

 membranous plates (PI. 28. fig. 2 g), tra- 

 versed by innumerable tracheae. In JEshna, 

 Libellula, and Calopteryx, the branchiae are 

 internal, the folds of the rectum being situ- 

 ated within that organ, which is powerfully 

 muscular (PI. 28. fig. 20, rectum of JEshna). 



In jEshna the branchial plates are nume- 

 rous, semicircular, horizontal, imbricated, and 

 arranged alternately in six regular and sym- 

 metrical columns. The laminae consist of a 

 network of fine tracheae, communicating 

 with those of the body, and situated beneath 

 the mucous membrane, which is fringed 

 around them with tubular papillae containing 

 prolongations of the tracheae, the free mar- 

 gin of each lamina being marked with a 

 brown crescentic spot. In this larva, some 

 of the dorsal segments are spinous, and 

 ocelli are absent. The end of the abdomen 

 is furnished with five moveable valvular 

 pieces (PL 28. fig. 29), three of which are 

 larger than the others, and the uppermost 

 of which is notched at the end. These 

 pieces, by their contraction, expel the water 

 from the rectum, by which it becomes re- 

 newed, this simultaneously effecting the 

 locomotion of the animal. 



The labium of^Eshna possesses a remark- 

 able structure, forming an elongated, some- 

 what spatulate, mask-like appendage, which 

 completely closes the mouth when unem- 

 ployed. It consists of a basal piece, uniting 

 it to the under side of the head ; an elon- 

 gated portion, somewhat dilated in front, 

 with the outer and anterior angles of which 

 is articulated a pair of somewhat triangular 

 pieces, furnished with minute blunt teeth 



along the posterior and the internal margin, 

 and with a strong curved and pointed claw or 

 tooth on each side, let into and articulated 

 with its anterior margin (PI. 41. fig. 16). 



In Libellula the six biserial rectal columns 

 are also present,but the papillae are absent, and 

 the intermediate external caudal appendage 

 is pointed, and not notched (PL 28. fig. 22). 



In Calopteryx the rectal branchiae are 

 more simple, consisting of three plates, at- 

 tached only by the end, and resembling in 

 structure the external plates of Agrion. In 

 Calopteryx the ocelli are distinct, and the 

 external caudal apparatus consists of three 

 channeled and keeled pieces. 



The spiracles of these larvae and pupae are 

 more or less concealed in the interspace be- 

 tween the proto- and mesothorax ; they are 

 transverse, bilabiate, and furnished with a 

 musculo- membranous valve. 



BIBL. Dufour, Ann. des Sc. nat. 1852. 

 xvii. 65 ; Westwood, Introduction, fyc. 



LIBER. The term liber-cells or liber- 

 fibres is applied to the very long forms of 

 prosenchymatous cells, occurring either iso- 

 lated or in bundles at the outside of the 

 cambium-layer of Dicotyledons, and often 

 in the pith and the ribs of the leaves; to the 

 cells of similar form and character occur- 

 ring in the outer part of the fibro-vascular 

 bundles of Monocotyledons, and in the 

 branches of these containing no spiral struc- 

 tures ; also to the fibrous cells of the same 

 kind found in the husks of many fruits, as of 

 the Cocoa-nut. No exact line of demarca- 

 tion can be drawn between liber-cells and 

 wood-cells, since the shorter of the former 

 pass into the latter. As a rule, they are 

 much thickened by secondary deposits (PL 

 38. fig. 27), but these deposits are tougher 

 than those of wood-cells, and while they 

 have pores, these are never bordered with a 

 rim. Mohl has shown that ordinary liber- 

 cells are composed of cellulose. Liber-cells 

 are found not unfrequently branched, and 

 this in cases where they appear to be evi- 

 dently genuine elementary organs ; some of 

 the branched forms, however, are said to 

 originate in the manner generally attributed 

 to MILK-VESSELS, namely by deposition in 

 intercellular passages. Hence has arisen a 

 difference of opinion, which has arrived at a 

 climax in the assertion of Schacht that all 

 milk-vessels are liber-cells, and all originate 

 from ordinary cambium-cells. 



The layers of thickening on the walls of 

 liber-cells frequently exhibit a spiral stria- 

 tion, especially after treatment with acids 



