Arthropod silk is among the most remarkable materials produced by animals, and is diverse in chemical composition and strand morphology. Different types of silk are produced by different spinning processes, and spiders show the greatest diversity in both. Extremely flattened silk strands with exceptional properties are spun by recluse spiders (Loxosceles spp.) using a single pair of spigots, but this spinning process remains unknown. We here combine movies, electron microscopy, and X-ray microscopy to investigate how Loxosceles laeta spin their peculiar silk. We found that L. laeta show a unique weaving behaviour: the two anterior lateral spinnerets move very rapidly (8.5–13 times per second), each spinning a single strand of silk that is held by the posterior spinnerets; the silk is never pulled with the legs. Outgroup comparison with closely related spider families reveals that recluse spiders have a highly modified spinneret morphology, with elongate anterior spinnerets, a diastema between the spinnerets, modified setae that possibly help holding the silk strands, and long and thin muscles associated to the spinnerets. In contrast to all other arthropods, these morphological specializations allow them to rapidly spin without using the legs or the substrate – revealing the first self-sufficient spinning apparatus in arthropods. This explains how a dense, sticky silk can be constructed from a single pair of spigots in a fast manner.
|jzo12462-sup-0001-MovieS1.mpgMPEG video, 3.9 MB||
Movie S1. Movie of an adult female Loxosceles laeta spinning its web while allowed to move freely in a Petri dish (24 frames per second).
|jzo12462-sup-0002-MovieS2.mp4MPEG-4 video, 10.9 MB||
Movie S2. Close-up movie of the spinnerets of an adult female Loxosceles laeta while spinning its silk at normal speed (60 frames per second); the specimen was being held in a glass tube and only the abdomen was able to move.
|jzo12462-sup-0003-MovieS3.mp4MPEG-4 video, 80.6 MB||
Movie S3. Close-up movie of the spinnerets of Loxosceles laeta while spinning its silk with speed reduced to 20% (this is modified from Movie S2; originally filmed at 60 frames per second, reduced to 12 frames per second).
|jzo12462-sup-0004-MovieS4.mp4MPEG-4 video, 48.2 MB||
Movie S4. Close-up movie of the spinnerets of a manipulated adult female Loxosceles laeta. The modified setae from the posterior spinnerets were removed with fine forceps; the specimen was held in a glass tube and only the abdomen was able to move.
|jzo12462-sup-0005-MovieS5.mpgMPEG video, 8.6 MB||
Movie S5. 3D reconstruction of the muscles associated to the spinnerets of an adult male Loxosceles laeta. Colours correspond to those in Fig. 4.
|jzo12462-sup-0006-MovieS6.mpgMPEG video, 35 MB||
Movie S6. 3D reconstruction of the muscles associated to the spinnerets of an adult female Scytodes globula. Colours correspond to those in Fig. 4.
|jzo12462-sup-0007-FigureS1-S3.pdfPDF document, 5.1 MB||
Figures S1–S3. Supplementary figures depicting dissections of spinneret muscles of fresh specimens (S1), SEM images of internal apodemes associated to spinneret muscles (S2), and SEM images of silk of the African species Loxosceles simillima (S3).
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