SCIENCE-GOSSIP. 



8i 



small scaly projecting plate immediately above 



and covering the base of the central anal-point 

 taking their place as a sexual feature in the 

 Libellulidae, whilst in the Agrionidae the atrophied 

 organs occur in their normal position as small 

 hooks. 



A somewhat nasty task is 

 that of securing specimens, 

 which usually have to be picked 

 out of the often fetid slime 

 forming the ground layer of 

 some dirty pond. The larvae 

 themselves are, however, very 

 cleanly, carefully washing their 

 faces and limbs after nearly 

 every meal. They are ex- 

 tremely voracious, and few 

 aquatic things are able to 

 escape them : the larger water- 

 beetles and the water-scorpions 

 are alone secure from attack. 

 The peculiar " mask " forming 

 the lower jaw is fairly well- 

 known to most people, and, 

 as Butler, in his " Pond Life," 

 says, " looks something like a 

 broad-handled ladle, attached 

 by the handle to the lower 

 part of the head." P. Amans 

 (Revue Montp. [3] 1) has 

 given some new and interest- 

 ing ideas on the mechanism. 

 These larva; are as a rule 



Fig. A. 



The index letters in italics accom- 

 panying fig. A are as follows:— a, 

 antennae ; aa, eyes ; b, occiput ; bb, 

 prothoracic-ridge ; c , thoracic-spira- 

 cles ; d, wing-sheath covers ; e, 

 wing-sheaths ; /, tubercles on median 

 line of segments ; g, caudal appen- 

 dages, 1, superior appendage, 2, 

 lateral appendages, 3, inferior or 

 posterior appendages ; h, femora ; 

 i, tibia and metacarpus ; /, tarsal- 

 joints. 



sluggish in their movements, 



lurking in mud or clinging to or crawling on 



water-plants. When alarmed or very hungry, 



Fig. P.. 



Fig. B shows /;, the mandible, attached to, and /, the 

 front border of, the mask. 



however, they move with surprising rapidity. 

 Their legs being ill adapted for swimming, 

 they glide through the water by a strange use of 

 their breathing apparatus, situated at the hinder 

 end of the body. A valved orifice surrounded by 

 five movable spinous processes allows of alternate 

 entrance and expulsion of water by rhythmic 



abdominal movemenl s. In swimming, a jet of water 

 is so violently expelled that by its action on the 

 surrounding body of water, the insect is sharply 

 propelled through the medium. In 1700, Dr. 

 Poupart sutfKested that this was a mode of defeca- 

 tion. These remarks, anent respiration, apply to 

 the Libellulidai. The Agrionine 

 larva: breathe by an entirely 

 different method, being fur- 

 nished with three leaf-like 

 appendages at the tail end of 

 the body, ramified in all direc- 

 tions by small tracheae, which 

 extract the oxygen from the 

 water ; constantly vibrating, 

 they also serve as locomotive 

 organs. In Agrion proper 

 {Calopteryx, Leach), the respira- 

 tion partakes of that common 

 to both Libellulida- and 

 Agrionida?, inasmuch as the 

 respiration is internal and 

 rectal as in the former, and 

 external and tracheal as in 

 the latter. This mode of 

 respiration is entirely con- 

 fined to the Calopterygian sub- 

 group, and, as Dufour says, 

 " establishes an enormous 

 anatomical distance between 

 Agrion and Ccenagrion." 



In the sub-family Coena- 

 grioninae, the rectal circulation 

 does not apparently exist. 

 There may, however, be eventually found some- 

 thing approaching this, as specimens frequently 

 lose their caudae, yet continue to live without 

 apparent discomfort, a greater degree of lamellar 

 oscillation being the only perceptible difference. 



There are also usually present a pair of thoracic 

 spiracles exposed to view in the Libellulidae, but 

 hidden in the Agrionidae. They have no direct 

 connection with the rectal circulatory system, 

 during the major part of the pupal life, performing 

 no function. A day or two prior to the emergence 

 of the imago, they are brought into action, the 

 about-to-be-metamorphosed nymph resting mean- 

 while on some plant or stone, the rectal circulation 

 being discontinued, and the thorax elevated above 

 the surface of the water. 



The initials in tig. C refer to the female valves ; m, of 

 Libellulina : n, of AEschnince, and 0, of Agriom '■:., . 



