Introduction. 



The Geometridae are one of the largest of the families of Lepidoptera, and are distributed throughout 

 the entire world so far as vegetation extends, a few species being found even in the extreme polar regions. It 

 is impossible to estimate at present, even approximately, the probable number of the species. Many new 

 species are found in every extensive collection that is brought fi-om New Guinea, Central Africa, South Ame- 

 rica, etc., and the better-known faimae are constantly being enriched by the discovery of new forms or especially 

 the differentiation of close allies which had previously been confused together. Ten years ago Staudinger- 

 Rbbel enumerated over 1230 Palearctic species, although excluding much of the south-eastern part of the 

 fauna; and at about the same time Dyar knew over 800 species as North American. For some other regions, 

 including the enormously rich countries of equatorial South America, no list at all exists, and if such were 

 issued today it would be foimd exceedingly incomplete tomorrow. There are, however, certain districts in 

 both hemispheres, including, for instance, much of the Argentine Republic, where Geometridae are comparative- 

 ly scarce, or appear to be far outnumbered by representatives of some other families, such as the Pyralidae. 



The family is on the whole a very natural one, and except possibly in Australia — where the most an- 

 cestral tj^es occur — quite sharply differentiated from all other families. The Epiplemidae, the Polyplocidae 

 or Cymatophoridae, the Notodontidae — all of which have been more or less associated with them by one or 

 another systematist — have no really near relation with them. They may be regarded as philogenetically of 

 rather recent origin, the larvae in particular having reached a very advanced specialization; and like other 

 recent, plastic groups they show a strong tendency to split up into a very large number of closely-alUed 

 species (for instance, in the genus Eupithecia), some of which are possibly still ,,in the making", e. g. Eupithecia 

 innotata, fraxinata and tamarisciata, E. denotata and jasioneata, Ectropis bistortata and crepuscularia, some of 

 the South American Nephodia (Nipteria), etc. Of fossil forms, scarcely half a dozen are known (see Hand- 

 lirsch. Die Fossilen Insekten), and these are of quite doubtful affinities. 



The Geometridae are generally recognizable by their build and aspect, apart from any structural dis- 

 tinctions. The comparatively slender body and ample wings, the latter in repose usually partially or almost 

 wholly extended and pressed out flat against the object on which they rest, give them a very distinctive appear- 

 ance, although occasionally the inexperienced collector mistakes for them certain slenderly-built Polyplo- 

 cidae, Drepanidae or the Epiplemidae, which share with them the general proportions. The Pyralidae are 

 less often (though still occasionally) confused with them ; the very general pearly gloss, differently folded hind- 

 wings and long legs of the last-named family are usually sufficient safequard against the danger of mistake. 

 Some Geometridae sit either habitually or occasionally with the wings erected over the back, after the manner 

 of a butterfly — a very rare attitude among the moths in general, except when drying their wings on emergence 

 from the pupa. The genus Selenia and several Larentiidae {Hydriomena, Ceratodalia, Euchoeca and others) 

 may be cited as examples. Very few indeed close the wings tightly around twigs {Brephos, Alsophila, Chesias, 

 etc.) or round the body after the manner of a Noctuid {Salpis, etc.); scarcely any, so far as is known, adopt 

 any very eccentric protective posture (Zamacra). 



In the struggle for existence the Geometridae rely very largely, both in the larval and imaginal stages, 

 on the simpler devices of protective resemblance, which are in most cases (especially as regards the larvae) 

 very beautifully perfected; that is to say, they generally assimilate very exactly, when at rest, to their natural 

 inanimate environment, and very seldom resort to warning coloration or to mimicry of nauseous or dangerous 

 insects or other living creatures. 



