adds an effect file for grubs

So they can grow similar to spiderlings

code/game/objects/effects/grubs_vr.dm

@@ -0,0 +1,127 @@
+/obj/effect/spider/grub
+	name = "grub larva"
+	desc = "It never stays still for long."
+//	icon = 'grub_vr.dmi'
+	icon_state = "spiderling"
+	anchored = 0
+	layer = 2.7
+	health = 3
+	var/last_itch = 0
+	var/amount_grown = -1
+	var/obj/machinery/atmospherics/unary/vent_pump/entry_vent
+	var/travelling_in_vent = 0
+
+/obj/effect/spider/grub/New(var/location, var/atom/parent)
+	pixel_x = rand(6,-6)
+	pixel_y = rand(6,-6)
+	processing_objects |= src
+	//60% chance to grow up
+	if(prob(60))
+		amount_grown = 1
+	get_light_and_color(parent)
+	..()
+
+/obj/effect/spider/grub/Destroy()
+	processing_objects -= src
+	walk(src, 0) // Because we might have called walk_to, we must stop the walk loop or BYOND keeps an internal reference to us forever.
+	return ..()
+
+/obj/effect/grubs/grub/Bump(atom/user)
+	if(istype(user, /obj/structure/table))
+		src.loc = user.loc
+	else
+		..()
+
+/obj/effect/spider/grub/proc/die()
+	visible_message("<span class='alert'>[src] dies!</span>")
+	new /obj/effect/decal/cleanable/spiderling_remains(src.loc) //Leaving this. It works.
+	qdel(src)
+
+/obj/effect/spider/grub/healthcheck()
+	if(health <= 0)
+		die()
+
+/obj/effect/spider/grub/process()
+	if(travelling_in_vent)
+		if(istype(src.loc, /turf))
+			travelling_in_vent = 0
+			entry_vent = null
+	else if(entry_vent)
+		if(get_dist(src, entry_vent) <= 1)
+			if(entry_vent.network && entry_vent.network.normal_members.len)
+				var/list/vents = list()
+				for(var/obj/machinery/atmospherics/unary/vent_pump/temp_vent in entry_vent.network.normal_members)
+					vents.Add(temp_vent)
+				if(!vents.len)
+					entry_vent = null
+					return
+				var/obj/machinery/atmospherics/unary/vent_pump/exit_vent = pick(vents)
+				/*if(prob(50))
+					src.visible_message("<B>[src] scrambles into the ventillation ducts!</B>")*/
+
+				spawn(rand(20,60))
+					loc = exit_vent
+					var/travel_time = round(get_dist(loc, exit_vent.loc) / 2)
+					spawn(travel_time)
+
+						if(!exit_vent || exit_vent.welded)
+							loc = entry_vent
+							entry_vent = null
+							return
+
+						if(prob(50))
+							src.visible_message("<span class='notice'>You hear something squeezing through the ventilation ducts.</span>",2)
+						sleep(travel_time)
+
+						if(!exit_vent || exit_vent.welded)
+							loc = entry_vent
+							entry_vent = null
+							return
+						loc = exit_vent.loc
+						entry_vent = null
+						var/area/new_area = get_area(loc)
+						if(new_area)
+							new_area.Entered(src)
+			else
+				entry_vent = null
+	//=================
+
+	if(isturf(loc))
+		if(prob(25))
+			var/list/nearby = trange(5, src) - loc
+			if(nearby.len)
+				var/target_atom = pick(nearby)
+				walk_to(src, target_atom, 5)
+				if(prob(25))
+					src.visible_message("<span class='notice'>\The [src] slides[pick(" away"," around","")].</span>")
+		else if(prob(5))
+			//vent crawl!
+			for(var/obj/machinery/atmospherics/unary/vent_pump/v in view(7,src))
+				if(!v.welded)
+					entry_vent = v
+					walk_to(src, entry_vent, 5)
+					break
+
+		if(amount_grown >= 100)
+			var/spawn_type = /mob/living/simple_animal/retaliate/solargrub
+			new spawn_type(src.loc, src)
+			qdel(src)
+	else if(isorgan(loc))
+		if(!amount_grown) amount_grown = 1
+		var/obj/item/organ/external/O = loc
+		if(!O.owner || O.owner.stat == DEAD || amount_grown > 80)
+			O.implants -= src
+			src.loc = O.owner ? O.owner.loc : O.loc
+			src.visible_message("<span class='warning'>\A [src] makes its way out of [O.owner ? "[O.owner]'s [O.name]" : "\the [O]"]!</span>")
+			if(O.owner)
+				O.owner.apply_damage(1, BRUTE, O.organ_tag)
+		else if(prob(1))
+			O.owner.apply_damage(1, TOX, O.organ_tag)
+			if(world.time > last_itch + 30 SECONDS)
+				last_itch = world.time
+				O.owner << "<span class='notice'>Your [O.name] itches...</span>"
+	else if(prob(1))
+		src.visible_message("<span class='notice'>\The [src] squelches.</span>")
+
+	if(amount_grown)
+		amount_grown += rand(0,2)