THE SILK-SPINNING HABIT IN BUTTERFLY L.ARVM. 51 



edge of the rachis. The threads each pass into one of the two grooves 

 surrounded by gum, and are pressed by the powerful contractions of 

 the muscles of the plates, so that each is compelled to mould itself in 

 the groove it occupies, and to take its shape. The spinneret thus 

 compresses the thread and diminishes its diameter, whilst the constant 

 compression of the thread as it passes through the press keeps it in a 

 certain state of tension so as to allow the caterpillar, while spinning, 

 to firmly hold its thread ; the press does not act directly on the silken 

 thread, but through the gummy layer which transmits over the whole 

 surface of the silken fluid the pressure exerted on it. After having 

 overcome this difficult passage, the silk thread has acquired its definite 

 form and passes out of the spinneret. [Full details of the structure 

 of the spinning organs, the formation of silk, the drawing of the 

 thread from the spinneret, etc., are to be obtained from Packard's 

 Textbook of Entomology, pp. 339 et seq.] These preliminary details will 

 perhaps enable the young lepidopterist to understand some of the 

 observations he is certain to make when watching the larvae of butter- 

 flies, for the silk-spinning habit is to be observed in a greater or less 

 degree in all butterfly larvae. 



It is remarkable how variable in degree is the silk-spinning habit 

 developed in the larvae of different butterflies. Usually, but not at all 

 necessarily, the larvae of allied butterflies have a somewhat similar 

 habit in this respect, e.g., the larvae of most Ruralids, and many 

 Pierids, spin very little silk, whilst those of others spin much. The 

 larvae of certain Vanessids, Melitaeids, etc., spin threads wherever they 

 walk, being apparently unable to move about in their early stages, at 

 least, without so spinning. All larvae, however, whatever be their general 

 habit, spin a silken web on which to fix themselves during the period 

 of moulting, the old larval skin being attached thereto before the larva 

 in its new dress withdraws itself therefrom. Some larvae construct 

 nests in common, and live gregariously, especially when young, others 

 fold up one or more leaves with silk, and live in the hollow thus 

 constructed singly ; others live quite exposed, resting on a leaf, stem, 

 or other part of the foodplant, and, at the most, spinning a few silken 

 threads as a pad on which to stand. 



As a rule, butterfly larvae live an exposed life. Of those that 

 hide, however, in the fashion of so many other lepidopterous larvae, 

 by making a tent of one or more leaves in which to secrete them- 

 selves, the Urbicolid larvae are the most remarkable. All our British 

 species do this, as will be seen by reference to our notes on the 

 " habits of the larvae " (posted,, pp. 98, 108, 120, etc.), and the 

 habit is common to the superfamily. There is no need to 

 redescribe the habits of our British species in this respect, but 

 we may note that Scudder, speaking of the American species, 

 says that " the ' skipper ' larvae form a nest of a single leaf, in 

 early life, folding over a little piece of leaf, and fastening the edge to 

 the opposite surface by a few loose strands of silk ; to effect this, they 

 first bite a little channel into the leaf, at just such a place as to leave 

 a fragment of leaf, neither too large, nor too small, to serve as a roof 

 when they shall have turned it over ; often they have to cut two 

 channels in order to procure a flap sufficiently small for their purpose ; 

 and it is curious to watch one of these tender creatures, just as soon as 

 it has devoured its eggshell, struggling with a tough oak-leaf, to build 



