16 INTRODUCTION 



incomplete, the Lepidoptera of many regions being as yet 

 very inadequately known. The geographical distribution of 

 each species within the British Isles is reckoned (unless other- 

 wise expressed) from the south northwards, and from the east 

 westwards, and stated in counties except in the case of Ireland, 

 which (being still very inadequately explored) is merely 

 quartered into north, south, east, and west ; England is taken 

 to include Wales, Britain to include England, Wales, and 

 Scotland ; limits are inclusive ; thus " England to York " means 

 " the whole of England and Wales from the south as far as and 

 including Yorkshire ; " " Kent to Dorset and Norfolk " means 

 the whole triangle of which these three counties form the 

 angles. The Clyde is taken to mean the latitude of the Firth 

 of Clyde. The geographical distribution outside the British 

 Isles is reckoned from the west eastwards, and from the north 

 southwards ; it is no doubt often incompletely known, especially 

 in the smaller species. A species is said to be common within 

 its range if it is found wherever its food is plentiful ; local, if 

 it is restricted, within the limits of its range, by other conditions 

 than those connected with the supply of food. Thus an oak- 

 feeding species is common if it occurs wherever oaks grow freely 

 inside its territory, local, if it occurs in some oak woods and not 

 in others. All species may be expected to be less plentiful 

 towards the outskirts of their range. 



Following the geographical distribution are numbers indicat- 

 ing the months (1 to 12) in which the imago is ordinarily to 

 be found; thus 6 is June, 10 is October. These dates must 

 be taken to refer to an average district of low elevation in 

 England, and an ordinary season ; great allowance must be 

 made for variation due to differences of situation, latitude, 

 elevation, and weather ; these differences may influence the date 

 by as much as a month or six weeks in either direction. The 

 dates for the duration of the larval stage are indicated in the 

 same way. Some species have normally two generations in the 

 year, and these are noted, but many others are liable to have a 

 second generation in warm localities and favourable seasons ; of 

 these irregular and variable appearances no account is taken. 



The larval descriptions are intended to refer to the larva in 

 its final stage, but whilst still feeding ; after the larva has 

 ceased feeding, a day or two usually elapses before it begins 

 to spin its cocoon, and during that time considerable changes 

 of colour frequently take place, probably mainly due to the 

 cessation in the supply of chlorophyll, since these changes are 



