sac spider (family Clubionidae)
Refers to any spider of the family Clubionidae. A technical zoological term; in everyday contexts, people may simply say クモ.
Sac spiders are known for making sac-like nests on the undersides of leaves.
Atypus karschi (Asian mygalomorph spider)
Specifically refers to the species Atypus karschi, a type of trapdoor spider. Cross-referenced with ジグモ, which is another name for a similar spider. This sense is distinct from the general sac spider meaning.
See also: ジグモ
Jigumo and fukurogumo are closely related, but are often treated as separate species.
Hiragana spelling; also used, but katakana is more typical for animal names.
Kanji spelling; rarely used in practice. This form but marks the word is listed as usually written in kana.
Compound of 袋 (fukuro, 'bag, sack') and 蜘蛛 (kumo, 'spider'), referring to the sac-like nests built by these spiders. The exact historical derivation is uncertain, but the name is descriptive.