LIBELLULHLE. 



[ 461 ] 



LIBER. 



BIBL. Ehrenb. Inf. 311 ; Duj. Inf. 458 ; 

 Stein, Inf. 184; Clap, et Lackm. In/us. 

 1868. 



LIBELLULlDvE. 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 pupoe, which live in water, and 

 are furnished with branchiae, either inter- 

 nal or external, situated at the end of the 

 body. External branchiae are seen in 

 Ayr ion (PI. 35. fig. 17), They consist of 

 three membranous plates (PL 35. fig. 2 g), 

 traversed by innumerable tracheae. In 

 jEschna, Libellula, and Calepteryx the 

 branchiae are internal and situated within 

 the rectum (PI. 35. fig. 20, rectum of 

 jEschna). In this last genus the branchiae 

 are in the form of plates, which are nu- 

 merous, semicircular, longitudinal, imbri- 

 cated, and arranged alternately in six regu- 

 lar and symmetrical columns. The laminae 

 consist of a network of fine tracheae, com- 

 municating with those of the body and 

 situated beneath the mucous membrane. 



The end of the abdomen is furnished with 

 five movable valvular pieces (PL 35. 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; it is 

 renewed ; and the expulsive force effects the 

 locomotion as well as the respiration of the 

 insect. The labium of *3$$chna also possesses 

 a remarkable structure, forming an elongated, 

 somewhat spatulate, mask-like appendage, 

 which completely closes the mouth when 

 unemployed (PL 50. fig. 16). In the other 

 genera the structure is very similar. In 

 Libellula six biserial rectal columns are also 

 present ; but there are no papillae on them ; 

 and the caudal appendage is pointed and 

 not notched (PL 35. fig. 22). In Calepteryx 

 the rectal branchiae are more simple, con- 

 sisting of three plates attached only by the 

 end, and resembling in structure the external 

 plates of Agrion ; the ocelli are distinct ; 

 and the external caudal aperture consists of 

 three channelled and keeled pieces. The 

 spiracles of these larvae and pupae are more 

 or less concealed in the interspace between 

 the proto- and mesothorax ; they are trans- 

 verse, bilabiate, and furnished with a mus- 

 culo-membranous valve. 



BIBL. Dufour, Ann. Sc. Nat. 1852, xvii. 



65 ; Westwood, Introd. ; Packard, Amer. 

 Natur. v. 1871 ; Oustalet, Ann. So. Nat. 

 xii. ; Selys-Longchamps, Mon. Libell. d' Eu- 

 rope ; Charpentier, Libell. Europ. 48 pis.: 

 Lenderfeld, Wings of, 1831. 



LI'BER (Phloem). The term liber-cells 

 or -fibres is applied to the very long pros- 

 enchymatous cells, occurring isolated 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 occurring in the 

 outer part of the fibro-vascular bundles of 

 Monocotyledons, and in the branches of those 

 containing no spiral structures ; also to the 

 fibrous cells of the same kind found in the 

 husks of many fruits, as the cocoa-nut ; they 

 are constantly ringed with sieve-cells or 

 -tubes. No exact line of demarcation 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 47. fig. 27) ; but 

 these deposits are tougher than those of the 

 wood-cells; and while they have pores, these 

 are never bordered with a rim. Liber-cells 

 are not unfrequently found branched (Rhi- 

 zophoraceae, Gmtum) ; and some of the 

 branched forms are supposed to originate 

 by the confluence of originally distinct cells, 

 after the manner of milk-vessels. 



The layers of thickening on the walls of 

 liber-cells frequently exhibit a spiral stria- 

 tion, especially after treatment with acids 

 (PL 28. figs. 2, 3, 25). Mohl has pointed 

 out the peculiar characters of certain struc- 

 tures associated with liber in the bark of 

 Dicotyledons and in the vascular bundles 

 of Monocotyledons. These are the vasa 

 propria, and are described under VASCULAR 

 BUNDLES. 



The importance of liber as a material for 

 textile fabrics has been spoken of under FI- 

 BROUS STRUCTURES, and examples cited ; 

 figures of various kinds of liber-fibre are 

 given in Plate 28. 



In Dicotyledonous stems the liber-fibres 

 are usually placed in large bundles opposite 

 the fibro-vascular bundles of the wood, 

 as in Urtica, Viscum, Clematis, Quercus, 

 &c ; j sometimes in small irregular groups, 

 as in Vinca and Linum ; in other cases they 

 stand in single rows, alternating with 

 parenchyma or vasa propria (Cupressinea 

 and Taxinece), while in many plants they 

 are irregularly scattered, as in Rhizophora, 

 Cinchona, Nerium, &c. Isolated liber-cells 

 occur in the pith of young shoots, and may 



