Ants

There isn't a rule. This is an example of the nonsensical state of English spelling.

The origin is Latin; -ant derives from the present participle of 1st conjugation latin verbs while -ent comes from the 2nd, 3rd and 4th conjugations. The matter is further complicated by passage of Latin originals through French to English. The French sensibly adopted -and for their spelling, and generally the English took that on. But around 1500 some academic snobs or other decided English spelling should reflect its true source and left us with the mess we have now.

So, the only way you can be sure of the 'correct' English spelling (apart from the exceptions, of course) is to learn Latin!

Here's what the online OED has to say:

-ant, suffix
a. Fr. -ant, sometimes:L. -entem, -ntem, -ntem, ending of pres. pple. (see -ENT); sometimes a later adaptation of -ntem only. All the participial forms were in OFr. levelled under -ant, the sole ending of the pr. pple., as L. amnt-, vidnt-, sednt-, crdent-em in Fr. amant, voyant, séant, croyant. But other words were subsequently adopted in their L. stem form, as prudent, présent, élégant. Hence Fr. words in -ant are of two kinds, one answering to L. -nt, the other to L. -ent, -nt. All were adopted, in their actual Fr. forms, in Eng., where they subseq. became -aunt; then again, with the change of stress, -ant, as L. affdent-em, diff-dent-em, plicnt-em, servient-em, tennt-em, OFr. afiant, defiant, pliant, serjeant, tenant, ME. afia(u)nt, defia(u)nt, plia(u)nt, serjeaunt, tenaunt. Most of them retain -ant, e.g. claimant, pleasant, poursuivant, servant, suppliant, valiant; but since 1500 some have been refashioned with -ent after L., wholly (as apparaunt, -ent), or partly (as in pendant, -ent, dependant, -ent, ascendant, -ent). Hence, inconsistency and uncertainty in the present spelling of many words, in which L. and Fr. analogies are at variance: see -ENT. Many new words of this class have been adopted from L. -ntem directly or through later Fr., or have been formed on L. analogies, or adopted from mod.Fr. and Romance -ant, -ante; as concomitant, protestant, commandant, anæsthesiant. For sense, see -ENT.

-ent, suffix
a. Fr. -ent, ad. L. -ent-em, the ending of pr. pples. of vbs. of the 2nd, 3rd, and 4th conjugation, as rdent-em, currentem, audientem. (In the pples. of the 3rd and 4th conjugation this ending represents OAryan -nt-, or perh. -ent-, of the ablaut-series -ent-, -ont-, -nt-; cf. Skr. -ant-, -at-, Gr. --, Goth. -and-, OE. -end-; in those of the 2nd conjugation it represents this suffix combined with the thematic -e- of the vb.; similarly the -ant- of the 1st conjugation includes a thematic -a-.) In OFr. this suffix and the corresponding -ant-em of the 1st conjugation were levelled under -ant, the sole ending of the Fr. pr. pple., as riant, courant, mourant, levant (:L. levantem). At a later time many L. forms in -ent-, which had acquired an adj. sense, were adopted in Fr. as adjs. with the -ent- unchanged, as diligent, évident; some of these were duplicates of living ppl. forms in -ant, as convénient = convenant, provident = pourvoyant, confident = confiant. The Fr. words in -ant, -ent, which were adopted into Eng., have generally retained the form of the suffix which they had in Fr.; but since 1500 there has been a tendency to refashion them after Lat., and hence several words in -ant have changed that ending for -ent, either entirely or in certain senses. In mod.Eng. also many Lat. words in -entem have been directly adopted, always in the form -ent. The conflict between Eng. and Fr. analogies occasions frequent inconsistency and uncertainty in the present spelling of words with this suffix; cf. e.g. assistant, persistent; attendant, superintendent; dependant, -ent, independent.
2. In sense the words in -ent, -ant are primarily adjs., sometimes distinctly ppl., as convergent, obsolescent, errant, peccant; some, however, are, like many words of the same type in Lat. and Fr., used as ns. (either in addition to the adj. use or exclusively), meaning (a) a personal agent, as agent, claimant, president, regent; (b) a material agent, as coefficient, current, ingredient, secant, tangent, torrent; esp. in Medicine, as aperient, astringent, emollient, expectorant.

Bugs Life

Answer:

Two classics from the 50's are THE NAKED JUNGLE based on Carl Stephenson's story "Leiningen Vs The Ants" and THEM featuring giant mutant man eating ants.

Arthropod

Although all 4 are types of animals they are quite different.

• An Arthropod is an invertebrate with an exoskeleton rather than an internal skeleton (and hence not a vertebrate since that would require having a backbone or the equivalent), a segmented body, and jointed paired appendages.
• A mollusk is also an invertebrate but has a mantle with a significant cavity used for breathing and excretion, a radula, a minutely toothed, chitinous ribbon kind of like a tounge, which is typically used for scraping or cutting food before the food enters the esophagus (except in the case of bivalves like clams and oysters, and a nervous system with two pairs of main nerve cords (one mostly serving the internal organs and the other the "foot") organized around a number of paired ganglia. In many species of mollusks the epidermis of the mantle secretes calcium carbonate and conchiolin which create a shell for the mollusk.
• The term "worm" applies to a disparate group of animals with long cylindrical tube-like bodies and no legs. Most, but not all, worms are invertebrates.
• Sponges are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them. They lack any nervous, digestive, or circulatory system. Most sponges rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. Obviously they live in the water since they rely on this water flow.