Hive Fleet Goliandr's existence raises more questions than it provides answers for. Indeed, the fleet seems to lack the base cunning and strategic thought that other Hive Fleets have demonstrated, moving from planet to planet seemingly at random, leaving fertile worlds half-eaten, or only to fly to an empty, barren moon several systems away.
The reasons for such behaviour are unknown, but the source of much speculation amongst those who study such things. Current theories range from the Tyranid Hive Mind not being as unified as first thought, to Goliandr being cut loose entirely from the larger Hive Mind, to the more sinister thoughts that the Hive Fleet is either looking for something specific, or running from something else. This strange behaviour has earned Hive Fleet Goliandr the nickname "The Mad Fleet".
What is known with certainty is that the bestial forces of Hive Goliandr make a point of attacking both Imperial and Heretic Space Marines whenever it encounters them. Furthermore, instead of eating the corpses of the slain, the servants of the Mad Fleet make a point of brutalizing and savagely attacking the corpses.
Indeed, it is not unheard of for the Mad Fleet to send task forces in search of Space Marines to kill, comprised of their largest and most dangerous creatures.
I've found a way of distinguishing the important bugs - namely by giving them a once-over with blood angels red before I get into the grey washes.
Biologically speaking, I don't know why they'd make the lead bugs visually distinctive, but then again this is the Mad Fleet, so logic doesn't necessarily always ring true for it in the first place.
More pics below!