83 
didae, and of insects susceptible to confiiiement, such as the Tenthri- 
dinidfe, are considered but briefly, and there are a few other points 
not treated in detail, for lack of time or space. It will be proper then 
for me to present, not a full discussion of methods, but certain notes 
on methods supplementary to the bulletin mentioned. My own per- 
sonal experience in the rearing of insects has been slight, and the facts 
which I shall give are derived solely from observation of the methods in 
use in the Division of Entomology, where they have been introduced by 
Prof. Riley, and where they are being carried on by his able and con- 
scientious assistant in charge of the insectary, Mr. Theo. Pergande. 
There is probably no group of insects more difficult to study than 
the Aphididae. Susceptible to changes of temperature and to excess 
or lack of moisture, attacked by a host of natural enemies, and pos- 
sessing generally alternate food plants, they are apt to foil the best 
meant endeavors to observe their life round. Dr. Riley has, however, 
mainly with the cooperation of Mr. Pergande, been able to fill out the 
numerous gaps in the life histories of many species and to record in 
full those of others new to biological literature. This has only been 
done by the patient labor of years and by the exercise of foresight and 
ingenuity to a most marked degree. Take the case of the Hop Plant- 
louse (Phorodon humuli), for example. The starting point in this study 
was the suspected, though not perfectly proved, identity of the form 
on Prunus and the form on Humulus. Nearly all of the observations 
were made in the field and very much in the following maimer: During 
the winter, winter eggs supposed though not known to be those of this 
species were marked by the hundred upon plum trees. With the burst- 
ing of the buds in the spring the marked eggs were examined. Hardly 
one out of a hundred was found to hatch, the others having been de- 
stroyed by predaceous insects or killed by the action of the weather. A 
number of stem-mothers were followed to their settling point : the leaf 
was marked, but at first was not inclosed m netting, as it was feared 
that this might interfere to some slight extent with the proper growth 
of the foliage. Each morning the offspring of each stem-mother were 
counted and were carefully removed with a camel's hair brush and 
placed upon adjoining leaves which were given characteristic markings. 
Thus the total number of offspring was ascertained and a supply of 
definitely known individuals of the second generation was provided 
for. In the same way the individuals of this second generation were 
followed, and as they began to give birth to their living young each one 
was visited every morning and the offspring of the previous day and 
night were removed, a sufficient number being stationed upon marked 
leaves to carry the investigation forward, while the rest were destroyed. 
At this stage of the investigation it became necessary to inclose indi- 
vidual leaves with netting in order to prevent the presence not only of 
predaceous insects but of other individuals of the plant lice which would 
prove disturbing elements and would be liable to confuse the investi- 
