Consider the gargantuan insect at the heart of this BBC clip, an African Dorylus queen:
She’s a strange beast. All army ant queens are.
Let’s contrast these insects with a queen whose robust form is representative of most other ant species:
Most army ant queens are long and slender, their thorax reduced, and they, like their workers, are either blind or nearly so. Technically, they are dichthadiigyne.
Army ant queens have two particularly stressful- and diametrically opposite- behaviors. First, they lay eggs. All the eggs. As army ant colonies can be huge (a few tens of thousands to 20 million or so individuals), and since workers don’t live long, queens pump out eggs at an enormous rate. Up to tens of thousands per day when they are in their laying cycle. Consequently, their abdomens must be large accommodate an extensive system of ovaries.
Second, queens have to be able to run in the frequent colony migrations. So they can’t merely be soft ginormous laying machines. They’ve got to be strong and streamlined, with tough walking legs. And unlike most ants whose queens fly to disperse, army ant queens have lost the flight apparatus and redesigned their thorax for dispersal by foot.
Thus, army ant queens are the product of a selective environment pulling in two directions at once: towards bloated egglayer, and towards distance runner. The result is a strangely armored wormy thing.