I can answer some of the below, as I was looking into that a few years ago. Spider was designed between 1969 and 1974 - the final lab report (#23) dates from December 1974. It was based around a serial loop running at T1 signalling speed (~1.5Mhz). Here is a video recorded by Dr. Fraser about it: https://www.youtube.com/watch?v=ojRtJ1U6Qzw (first half is about Spider, second half about Datakit). It connected to its hosts via a (discrete TTL-based) microcontroller or “TIU” and seems to have been connected almost immediately to Unix systems: the oldest driver I have been able to locate is in the V4 tree (https://minnie.tuhs.org/cgi-bin/utree.pl?file=V4/nsys/dmr/tdir/tiu.c). It used a DMA-based parallel interface into the PDP11. As such, it seems to have been much faster than the typical Datakit connection later - but I know too little about Datakit to be sure. There is an interesting visit report from 1975 that discusses some of the stuff that was done with Spider here: https://stacks.stanford.edu/file/druid:rq704hx4375/rq704hx4375.pdf Beyond those experiments I think Spider usage was limited to file serving (’nfs’ and ‘ufs’) and printing (’npr’). It would seem logical that it was used for remote login, but I have not found any traces of such usage. Same for email usage. From what little I know, I think that Datakit became operational in a test network in 1979 and as a product in 1982. “Network Unix” (https://www.rfc-editor.org/rfc/rfc681.html) was written by Steve Holmgren, Gary Grossman and Steve Bunch in the last 3 months of 1974. To my best knowledge they used V5 and migrated to V6 as it came along. I think they were getting regular update tapes, and they implemented their system as a device driver (plus userland support) to be able to keep up with the steady flow of updates. Greg Chesson was also involved with this Arpanet Unix. As far as I can tell, Arpanet Unix saw fairly wide deployment within the Arpanet research community, also as a front end processor for other systems. A few years back I asked on this list why “Network Unix” was not more enthusiastically received by the core Unix development team and (conceptually) integrated into the main code base. I understood the replies as that (i) people were very satisfied with Spider; and (ii) being part of Bell they wanted a networking system that was more compatible with the Bell network, i.e. Datakit. == In my opinion both “Spider Unix” and “Arpanet Unix” threw a very long conceptual shadow. From Spider onwards, the Research systems viewed the network as a device (Spider), that could be multiplexed (V8 streams) or even mounted (Plan9). The Arpa lineage saw the network as a long distance bidirectional pipe, with the actual I/O device hidden from view; this view persists all the way to 4.2BSD and beyond. I often wonder if it was (is?) possible to come up with a design with the conceptual clarity of Plan9, but organised around the “network as a pipe” view instead.