1898.] ENTOMOLOGICAL NEWS. 1 17 



moral of it all is that sensible error may frequently arise from 

 apparently trivial and wholly unsuspected causes, for in practice 

 I have always been careful to place the scale as close as practi- 

 cable to the object. 



With the micrometer scale described above it is the easiest 

 possible thing to measure accurately the minutest detail of an 

 insect, the value of a division with the ^j-inch objective being 

 .059 mm., a tenth of which can be readily estimated. I find, for 

 example, the diameter of the minute reticulations of the elytra 

 in a small Discoderus before me to be .012 mm., or .2 of a divi- 

 sion. In a truly scientific description a correct procedure would 

 demand a statement of the actual dimensions of all the parts, 

 exactly as in describing the skull of a mammal. The descriptions 

 which we give to-day will probably be considered absurd a few 

 centuries hence. 



I read with interest the article — vol. viii, p. 49 — regarding the massing 

 of Coccinnelids on the summit of Moscow Mt. Idaho. Here in Los An- 

 geles County, California, I have often seen two species of Lady-bird 

 beetles : Megilla vittigera and Hippodamia ambigua so clustered that 

 they could be gathered by the pint. I believe they cluster to protect each 

 other from the cold. It is their wont to seek crevice or cranny and in 

 betaking themselves to forked branches or space between sheathing blade 

 and slim of palm, they have touched and learned of and enjoyed the 

 warmth, and so the habit of piling up as we frequently see them here. 

 The cold nights here, so in contrast to the warm sunny days, I think may 

 have gendered this habit. 



Since coming to California. I have witnessed one exceeding migration 

 of our common thistle butterfly, Vatiessa {Pyrameis) cardui. This cos- 

 mopolite was flying all day in great flocks and all in one direction. I saw 

 one such migrating flight, if such it may be called, of the common milk 

 week butterfly, also a cosmopolitan species, Danais archippus, in Michi- 

 gan. The numbers in that case were very great, but not comparable to 

 the one of the other species seen here in Southern California. 



There is another insect that fairly swarms here in this section each 

 season. It may well be called the prune beetle, as it often entirely defo- 

 liates whole prune orchards. It is Serica mixta. Its handsome con- 

 gener, Serica fimbriata, is larger, comes earlier, but in far less numbers. 

 It is well that the latter comes late in August and September, else it would 

 do serious harm. In habits and appearance it reminds one of the May 

 beetle, Lachnosterna fusca, of the East in miniature.— A. J. Cook. 



