I've set up a spider slik factory (REGULAR cave spiders, not giant ones, I must point out), basicly consisting of a room where I've released a bunch of cave spiders my trapper had caught. Now, since they are vermin, I tried my best to make this work. The room is dug out in the mountain, with no corridors or other rooms above or below on the adjacent z levels, plus I rewalled and refloored the entire room with constructions. The entry way is a line of 10 adjacent doors, all set to no pet access (not that I had any cats until some stupid migrant brought one along, but I thought it might help).
All seemed well. I let out the 8 spiders I had captured and they went to work building a whole bunch of webs. It only took a couple of months for almost every tile in the room to be covered, which is well over 100 webs, and I harvested a few of them without any problem. However I went back to check if they'd replaced the ones I'd harvested and all of a sudden theres not a spider to be seen.
So... any ideas why? As far as I know, vermin don't need to eat (like all animals) and I would have thought cave spiders would have been quite at home in the mountain. I heard that some vermin hibinate (or vanish at least) over winter, but its mid summer. I'm not sure if vermin have a lifespan, which would make sense as a probable cause of thier sudden vanishing, but would this mean eventually every spider on the map will die, or is there some way they get 'spawned'?
On a different note, if anyone wants it I'll try and see if I can work out how to get the seed out of this map, because I seem to have struck it lucky in this location. I have my embark screen to show magma, but not any other features, but on this map I hit a (fairly tiny) open air chasm AND almost right next to the magma vent I've hit the top of an Adamantine vein. Plus the map biomes have sand, sedimentary layers with magnatite showing, marble and a brook up at the top of the mountain. Its easily the best location I've ever had, so I'm really trying not to screw it up hehe.
EDIT: A file export later and heres the seed.
Created in DF v0.28.181.40d.
[WORLD_GEN]
[TITLE:LARGE]
[SEED:883996304]
[HISTORY_SEED:3513537844]
[NAME_SEED:1923080024]
[DIM:257:257]
[END_YEAR:1050]
[BEAST_END_YEAR:300:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:1:400:800:800]
[RAINFALL:0:100:400:400]
[TEMPERATURE:25:75:400:400]
[DRAINAGE:0:100:400:400]
[VOLCANISM:0:100:400:400]
[SAVAGERY:0:100:400:400]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[GOOD_SQ_COUNTS:100:1000:2000]
[EVIL_SQ_COUNTS:100:1000:2000]
[PEAK_NUMBER_MIN:50]
[OCEAN_EDGE_MIN:2]
[VOLCANO_MIN:25]
[REGION_COUNTS:SWAMP:1032:7:6]
[REGION_COUNTS:DESERT:1032:7:6]
[REGION_COUNTS:FOREST:4128:13:12]
[REGION_COUNTS:MOUNTAINS:8256:9:9]
[REGION_COUNTS:OCEAN:8256:7:6]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:8256:13:12]
[REGION_COUNTS:HILLS:8256:13:12]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:400:400]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:100]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_RIVER:2]
[SHOW_EMBARK_POOL:2]
[SHOW_EMBARK_M_POOL:2]
[SHOW_EMBARK_M_PIPE:2]
[SHOW_EMBARK_CHASM:2]
[SHOW_EMBARK_PIT:2]
[SHOW_EMBARK_OTHER:2]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:10]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:8256:16512:8256]
[RAIN_RANGES:8256:16512:8256]
[DRAINAGE_RANGES:8256:16512:8256]
[SAVAGERY_RANGES:8256:16512:8256]
[VOLCANISM_RANGES:8256:16512:8256]
... and the location. I don't think thats the exact embark area I'm on, but it definatly covers all the features.