NORTH AMERICAN ODONATA. 159 



of the hind lobe varies in different species and sexes of the Agrioninae, 

 and in different genera of the Libellulinse. The pleura are quite 

 distinct and consist of two or three sclerites. At its anterior edge 

 each pleuron bears a rounded tubercle which slides in a groove on 

 the side of the posterior cephalic foramen. The sternum is of mod- 

 erate size; the first coxte are placed near its hind end. Internally, 

 on the floor of the prothorax are two chitinous projections, between 

 which the nerve-cord runs ; these, as well as some dorsal, downwardly- 

 directed processes, also serve for muscular attachments. 



The remainder of the thorax is characterized by its general ob- 

 liquity and the close union of meso- and metathorax. The obliquity 

 is due to the great development of the mesepiderna which meet each 

 other on the mid-dorsal line, in front of the tergal sclerites of the 

 same segment.* In consequence, the words " dorsum" and " tergum" 

 applied to the thorax have very different meanings. " Dorsum of 

 the thorax," as used by descriptive writers on the Odonata, and as 

 used in this paper, includes the mesepisterna and the meso- and meta- 

 thoracic terga, that is, all those sclerites which make up the back 

 of the thorax. So, also, " sides of the thorax' ' is equivalent to the 

 pleura of meso- and metathorax, less the mesepisterna. The terga 

 of these two segments taken together are often referred to as the inter- 

 alar space. Each tergum is made up of scutum, scutellum and post- 

 scutellum, in the Agrionidoe at least, while the prtescutum is absent, 

 or at least not externally visible, in the view of Packard (1). 



The suture at which the two mesepisterna meet on the dorsum is 

 usually raised into a ridge, the mid-dorsal thoracic carina (arete meso- 

 thoracique, Rambur). Each raesepisternum terminates on its outer 

 side at the humeral sidure, present in all Odonata, running from 

 beneath the base of the front wing to the hind edge of the second 

 coxa. A short distance above the secon4 coxa, a transverse suture 

 runs to the anterior opening of the mesothorax, so that a sclerite is 

 formed between the anterior mesothoracic opening on the inside, the 

 humeral suture on the outside, the second coxa below and the trans- 

 verse suture above — known as the mesinfraepistenium. 



It is apparently only in the Calopteryginse that the suture sepa- 

 rating meso- and metathorax is complete on the sides of the thorax. 



*■ Such is Packard's (1) view. Walsh (Proc. Ent. Soc. Phila. II, p. 217, 1863) 

 previously advanced the same opinion. Brauer, however, holds (Verhd. k. k. 

 zooi.-bot. Gesell. Wien sci, p. 355, 1885), this dorsal part to be mesothoracic 

 prescutum. See the discussion in the text, post. 



TEANS. AM. ENT. SOC. XX. JULY, 1893. 



