Here is the bit in question, out of the 28th chapter of the 29th book of Pliny's Natural History (in the Teubner edition of 1906 at least; my dictionaries give the 4th chapter instead).
tertium genus est eodem phalangi nomine araneus lanuginosus, grandissimo capite, quo dissecto inveniri intus dicuntur vermiculi duo adalligatique mulieribus pelle cervina ante solis ortum praestare, ne concipiant, ut caecilius in commentariis reliquit. vis ea annua est. quam solam ex omni atocio dixisse fas sit, quoniam aliquarum fecunditas plena liberis tali venia indiget.
And here is a translation by me.
A third kind [of creepy crawly] also by the name phalangium is a downy spider with a very grand head, inside which, when dissected, two worms are said to be found and, being attached to women before dawn using a deer skin, to make it so that they don't conceive, as Caecilius has attested in his commentaries. This power lasts a year. It is this alone which out of all the contraceptives it is good to have mentioned, because the fecundity of some women, abounding with children, wants such an indulgence.