BY R. J. TILLYARD. 627 



wings is retained in some genera (e.g. Leto, Text-fig. 81) and 

 quite lost in others (e.g. Trictena, Text-fig. 82) . 



In constructing the Archetype of this Order, the only diffi- 

 cult points to be decided are (a) the archetypic condition of the 

 media in the hindwing. and (6) the question of including certain 

 cross-veins. 



With respect to the media, we have to show, firstly, that a 

 separate M 4 is never present in this wing, and, secondly, that 

 the cross- vein marked m-cu is really that cross-vein, and not 

 the reduced basal piece of M 4 after fusion with Cu, as in the 

 Lepidoptera. 



A survey of all the known genera of the Order shows that a 

 separate vein M 4 is absent in the hindwings of all except the 

 two rather highly specialised genera Helicopsyche and Saeto- 

 triclia (Text-fig. 72), belonging to the family Sericostomatidae. 

 In the hindwings of these two genera, Ulmer (33) has considered 

 the three veins present as M 1+2 , Ms and M 4 respectively, on 

 the ground that the apical fork lies between the last two of them; 



Text-Fig.72. 

 Condition of the branches of the media in a, forewing and b, hindwing 

 of Saetotricha ptychopteryx Br. (fam. Sericostomatidae), to show 

 transference of M-2 across to M3. Lettering as on p. 535. 



whereas, in the hindwings of other Trichoptera, it lies between 

 the first two. The mistake in this is easily seen by comparing 

 the fore and hind wings together, when it will be clear (Text-fig. 

 72 I that, in the forewing, M 2 has migrated across from its ori- 

 ginal position, as the lower branch of M 1+2 to ^3, and has 

 thus carried the apical fork along with it. The same specialisa- 

 tion has evidently happened in the case of the hindwing; but, 

 since there is no separate M 4) the true homologies of the three 

 veins became obscured thereby. In Ulmer's notation, the apical 



