180 



PHILIP P. CALVERT. 



The tJionicic qnracles are considerably larger than the ahdoniinal. 

 Of the two pairs, the more anterior are the mesostigmata, which are 

 situated, one in the fore part of each niesepisternum. Each niesos- 

 tigma (see PI. II, fig. 13) is a narrow, transversely elongated open- 

 ing and from its position is often partly concealed by the hind lobe 

 of the prothorax. Within its lips is a chitinous, comb-like piece, 

 first described by Landois (Zeit. Wiss. Zool. xvii, p. 167, 1866) as a 

 sound-producing organ. The number of teeth of the comb varies 

 for diflTerent species and their length is not the same in different parts 

 of the comb. Between the larger teeth is a folded membrane con- 

 sisting of a single layer of rather larger cells. The second pair of 

 thoracic spiracles are the metastigmata. Each metastigma lies on the 

 side of the thorax in the lower part of the metepisternum ; it is 

 shorter, but wider than the mesostigraa, is without a chitinous comb, 

 but is guarded by a chitinous flap whose surface is spinous ; probabl}' 

 the comb of the mesostigma is merely a modification of such a flap. 



There are thus ten pairs of spiracles altogether, all functional. 



Both Landois I. c. aud Kranclier (Zeit. Wiss. Zool. xxxv, pp. 551-3) rejiaid the 

 thoracic spiracles as belonging to pro- aud mesothorax respectively; the latter 

 describes the metastigma and the abdominal spiracles in detail. There is ap- 

 parently no reason for thinking that the niesostigmatic comb does produce sound. 



The preceding account of the distribution of the trachefe is based on personal 

 dissection of Libellula pidchella. The writer is not aware that information exists 

 as to the variations which may occur between the imagos of different groups. 

 The accouut given above should be compared with the summary of the results 

 of various students who have treated of the Irachete of the nymphs (see post). 

 The writer is not acquainted with the contents of Dr. Palmen's "Morphologic 

 des Tracheen-Systems," a work which he has not seen. 



The respiratory movements (Barlow 10, Plateau 11) consist in an 

 alternate expansion (inspiration) and contraction (expiration) of 

 the abdomen. The expansion or contraction is sin)ultaneous in all 

 the segments, the length of the abdomen is not altered. The rate 

 of movement* varies greatly at different times owing to unknown 

 causes, but is always quickened by exercise, increased temperature, 

 galvanism and mechanical irritation ; the last three agents quicken 

 the movements in the decapitated insect as well. To adopt the con- 



* In au entire Libellula cyanea held in the hand, the inspirations wei'e 73 per 

 minute; similarly in a Plathemis trimaculata, 105-118 per minute. Another in- 

 dividual of this latter species immediately after alighting upon a fence-rail, 

 inspired 70 times per minute. 



