PUPA OF PRIVET HAWK MOTH. 155 



port ; whilst Herold has worked them out in Pieris Brassicae, as well also as the 

 changes undergone by the digestive system and the evolution of the male and 

 female organs. 



In some pupae, e.g. of Cossus, the edges of the abdominal somites are fringed 

 with short spines or adminicula to aid the movements of the animal. The apex of 

 the cremaster varies much in character. The duration of the pupal state differs 

 much. In small species it lasts only a few days. In Lepidoptera with two broods in a 

 year, e. g. Papilio Machaon, the pupal state of the first brood lasts thirteen days, of the 

 second from September to June. In the broods of various Pieridae, &c., the same 

 differences may be observed. The pupal state of Sphinx Ligustri is occasionally ex- 

 tended for a year beyond the normal : and this is often the case in some other 

 genera. It is a very common thing for a larval Tenthredinidan (Hymenoptera) to 

 delay assuming the pupal state within its cocoon, but it is an extremely rare occur- 

 rence in Lepidoptera. It is said, however, that if the Cossus larva makes its cocoon 

 in autumn, the caterpillar does not become a pupa till after winter has passed : if it 

 makes it in June, it becomes a pupa at once and emerges as an imago in three to 

 four weeks. 



Note. The wings make their first (i. e. outward) appearance in the pupa stage 

 of insects with a perfect metamorphosis. In the newly-formed Lepidopterous pupa 

 they are hollow sacs with a cavity continuous with the coelome. These sacs swell 

 out as the larval skin is being cast. But wings appear from the first as external 

 processes, gradually increasing in size with successive moults in insects such as 

 the Earwig, Ephemeron, Cockroach, i.e. in Insecta Ametabola^ and Hemimetabola. 

 Dewitz (Berlin. Entomol. Zeitung, xxv. 1881) has found in very early stages of 

 larval Trichoptera and Lepidoptera a pair of small meso- and meta-thoracic involu- 

 tions of the hypodermis cells containing an internal chitinous lamella continuous 

 with the cuticle. These involutions increase in size at every successive moult, and 

 acquire a more or less perfect investment of mesodermic cells derived from the 

 sheaths of either the tracheae or the nerves. They are evaginated previously to 

 the last moult by the withdrawal of the internal chitinous lamella, and when the 

 last larval skin is stripped off they appear as external sacs (supra). 



The homology and first origin of wings are points of great obscurity. There 

 are no traces of them in the most primitive Insectan order known, the Thysanura, 

 a survival of forms existing apparently before the acquisition of wings. Wings are 

 to be considered as secondary or acquired structures. It is tempting to regard them 

 as modified tracheal gills, which they much resemble in structure, and which are also 

 organs secondarily acquired. But it is impossible to suppose that all Insecta with 

 a very limited number of exceptions are descended from ancestors which took 

 to aquatic habits ; { took] because the first Tracheata were without doubt terrestrial 

 forms. And the only supposition that appears feasible is that respiratory structures 

 similar to tracheal gills were of use to terrestrial Insecta living under conditions 

 long passed away. The larval Calotermes rugosus (F. Miiller, J. Z. ix. 1875), one of 

 the Termitidae, animals of subterranean habit, develops peculiar dorsal appendages 

 devoid of tracheae on the pro- and meso-thorax. The pair on the prothorax dis- 

 appears : that on the mesothorax acquires tracheae, and grows into the mesothoracic 

 pair of wings. The metathoracic pair of wings develops in a similar manner but at 

 a later period. It must be remembered that the oldest fossil Insecta known, even 



