XXII 



EXTERNAL STRUCTURE 



491 



the insertion of the front wing. This stigmatic lobe is frequently 

 fringed with short hairs. 



The mesothorax is the largest of the three divisions of the 

 thorax proper ; its notum is large, and usually divided into two 

 parts by a transverse suture. The side- 

 pieces are so placed that the epimeron is 

 rather behind than below the episternum. 

 The mesosternum forms the larger part of 

 the under-surface of the alitrunk. A very 

 large phragma projects from the meso- 

 thorax into the interior of the body. The 

 mesothoracic spiracle is usually not visible ; 

 its existence was unknown to the older 

 entomotomists, who were in consequence led 

 to consider the spiracle of the median seg- 

 ment as belonging to the thorax. The meso- 

 thoracic spiracle is, however, easily seen in 

 Cimhex, placed in the suture between the 

 mesothoracic epimeron and the metathoracic 

 episternum, a little below the insertion of 

 the front wing ; close to this spot the meso- 

 phragma, just spoken of, comes, in Cimhex, 

 to the surface. The mesothoracic spiracle 

 is generally conspicuous in the worker ant. 

 The parts of the metathorax are usually small, but so much variety 

 prevails in this respect that no general description can be given. 



The structure of the posterior part of the alitrunk has given 

 rise to an anatomical discussion that has extended over three- 

 quarters of a century,^ with the result that it is now clear that 

 the posterior part of what appears to be thorax in Hymenoptera 

 is composed of an abdominal segment. This part has been called 

 " Latreille's segment," the " median segment," and the " pro- 

 podeum." The latter term was proposed by Newman, under the 

 form of propodeon,^ and appears to be on the whole the most 



Fig. 335. Articulation of 

 front legs of the hornet 

 ( Vespa crabro, ? ). A : 

 a, side -piece of pro- 

 thorax overlying the 

 prosternum ; b, coxa ; 

 c, trochanter. B, pro- 

 sternum proper, as seen 

 from front when ex- 

 tracted. 



1 For a history of this complex question, see Gosch, JVatu7'hist. TidsTcr. (Rk. 3) 

 vol. xiii. 1881 ; and also Brauer, Sitzh. Ak. Wien, Ixxxv. 1882. 



2 Introd. hist. Insects, 1841, p. 143. The names proposed by Newman may be 

 adopted when it is specially requisite to use terms that are morphologically correct. 

 According to his nomenclature the true whole abdomen of petiolate Hymenoptera 

 consists of three anatomical parts: 1, the petiole or podeon ; 2, the propodeon or 

 part in front of the petiole ; 3, the metapodeon or part behind the petiole. 



