This paper is based on the examination and identification of all the larva and adult Elateridæ occurring in soil samples taken from 1871 fields in the Mid- and West Wales province. All the species found are listed, together with some additional species discovered by general collecting in the Aberystwyth district.
The most important Elateridæ are Agriotes obscurus, A. lineatus, A. sputator and Corymbites cupreus. Athous larvæ are not particularly common, and only 15 fields were noted in which the population of Athous larvæ exceeded 100,000 per acre. In isolated instances Corymbites tessellatus, a small species of Corymbites of uncertain identity, and Cryptohypnus riparius occurred in numbers sufficient to constitute a possible danger to crops.
There is little definite relation between the total Elaterid population and the altitude of the land sampled; considering the province as a whole there is an indefinite tendency for the population to rise with increase in altitude. Its composition changes with elevation, particularly in fields over 1000 ft., where the relative proportion of the population made up by the three common species of Agriotes drops from the 95·4 per cent. characteristic of lowland fields to 62·8 per cent. In such fields Agriotes is extensively replaced by species of Corymbites, particularly C. cupreus; in a high proportion of fields over 1000 ft. and in many fields with an altitude of 600–1000 ft. cupreus is probably an important pest.
Of the three common species of Agriotes, obscurus is generally the most abundant and sputator the least, as far as can be gathered from records of adult beetles occurring in the soil samples. Obscurus is the dominant species at all altitudes, particularly in high ground, while sputator appears to be a lowland species, becoming rare in fields over 600 ft. and absent above 1000 ft.
The average total Elaterid population for the area is 218,800 per acre. 117 fields with populations of 600,000 to 999,000 were recorded, and only 28 with populations of 1,000,000 and above. There is some evidence suggesting that with increase in the total population there is a tendency for Agriotes to form an increasing proportion of the population to the exclusion of other species, and also some evidence suggesting that fields of high population tend to be grouped together.