356 Proceedings of the Royal Society of Edinburgh. [Sess. 
XXI. — Contributions to the Knowledge of the Family Cher- 
mesidse. No. I. : The Biology of the Chermes of Spruce 
and Larch and their Relation to Forestry. By H. M. 
Steven, B.Sc., Carnegie Research Scholar in Entomology, The 
University of Edinburgh. Communicated by Dr R. Stewart 
MacDoxjgall. 
(Read May 25, 1917. MS. received June 13, 1917.) 
CONTENTS. 
Part 
I. — Introductory : — 
PAGE 
Part III. — Relation to Forestry: — 
PAGE 
1. 
Introduction .... 
356 
1. Methods of Infection 
373 
2. 
Nomenclature of the Genera- 
2. Damage to Spruce . 
374 
tions ..... 
358 
3. Damage to Larch 
374 
3. 
Previous Research . 
360 
4. Control ..... 
375 
4. 
Technique . 
363 
Part IV. — General : — 
Part II. — Biology : — 
1. Theories as to the Origin of 
1. 
Genus Chermes s. str. 
365 
Migration .... 
378 
2. 
Genus Gnaphalodes . 
368 
2. Literature .... 
379 
PART I.— INTRODUCTORY. 
1. Introduction. 
Extensive and intensive research on the Chermes group has been 
carried out on the Continent during the last ten years, but there has 
been no close study of this group in Britain. This paper is a record of 
introductory research in this country on this complex but interesting and 
important group. 
These Aphids possess several well-marked features, namely : — 
A. Only conifers serve as food-plants. 
B. Two host-plants and a period of two years are normally required 
for the complete cycle of a species, and several parts of each 
host are utilised for food purposes. This has resulted in a well- 
marked polymorphism. 
C. If two hosts are required, one is constantly a species of the genus 
Picea, on which a gall is produced. The other may be a species 
of Larix, Pinus, Abies, or Pseudotsuga, but no gall is produced 
on any of these Genera. 
