> They look very different from one another.  **astice** as having a .. blue colour ...**aragosta** ..
> reddish body, **five pairs** of legs, ... 

>To me it looks like a giant
> **insect**.   

In English, however, they are both called lobster....
> I were to invite an Italian friend to a Boston
> restaurant and order lobster for both of us, she might be dismayed at
> seeing an astice on her plate.

Hi, Mary-Lou, in your question you have all the elements you need to explain the *mystery*: let's start with your correct impression of  a *giant insect* you inherited from your Latin ancestors:

The English common name 'lobster' is, as you quote, 
> a corruption of Latin [**locusta**](https://en.wiktionary.org/wiki/locusta#Noun_3)  f (genitive locustae); first declension:
 
1.   **locust,** grasshopper
2.    **crustacean**, marine shellfish, lobster

So, the Latins thought that *crustacean shellfish* looked like  *giant insects* , crickets, grasshoppers, locusts etc..., and, though the English words appear different now, they are both an adaptation of the same word ' locusta' => OE *lopustre* => *lop[y]ster* , and with *the voicing* of  /p/, eventually  = **lobster** .  (Source: Shorter Oxford Dictionary )

The Italian word ['aragosta'](http://www.treccani.it/vocabolario/aragosta/) derives from the same Latin word through the agglutination of the article [la locusta] => l'alocusta => aragosta. (an inverse phenomenon occurs in "all' amatriciana => alla matriciana")

The second element you need is taxonomy: the Italian language is rich in *common names* because it's so easy to italianize Linneus' Latin terms of scientific classification: therefore you have : aragosta, astice, omaro, palinuro, gambero di mare etc, and can distinguish families and species., the English language overcomes this handicap using qualifiers. That's almost all, but let's examine the problem in detail:

#order#
(..*.five pairs of legs*...) ,The order in question is 

- [Decapoda](https://en.wikipedia.org/wiki/Decapoda), and an *infraorder* is 
 - [Astacidea](https://en.wikipedia.org/wiki/Astacidea)  (from which you derive 'astice') which includes five *superfamilies*, one of these is:
  - [Nephropoidea](https://en.wikipedia.org/wiki/Nephropoidea), which contains the *families*

- [**Nephropidae**](https://en.wikipedia.org/wiki/Lobster), or **Homaridae**, and
- [**Palinuridae**](https://en.wikipedia.org/wiki/Spiny_lobster), or '*spiny*/ *rock* lobsters'

#genera#
- the genus [**Homarus**](https://en.wikipedia.org/wiki/Homarus#Species) (from which you derive 'omaro') includes  two species: " the *American* lobster (H. americanus) and the *European* lobster (H. gammarus), they are very similar and may have speciated as recently as the Pleistocene, during climatic fluctuations.The best characters for distinguishing them are the geographic distribution, with the American lobster in the western Atlantic and the European lobster in the eastern Atlantic, and by the presence of one or more teeth on the underside of the rostrum in the American species"

 - the 'American lobster'  is also known as *true* lobster, *Northern* lobster, or *Maine* lobster and it is not true that it is  only [red in colour](https://it.wikipedia.org/wiki/Homarus_americanus#/media/File:Lobster.jpg), in Italian: 'astice americano'

 - the 'European lobster' , or 'common lobster'  is called in Italian : 'astice' or 'astice europeo'

- [**Palinurus**](https://en.wikipedia.org/wiki/Palinurus_%28genus%29) is a genus in the family of *Palinuridae*, and
 - [**Palinurus elephas/ vulgaris**](https://en.wikipedia.org/wiki/Palinurus_elephas) is "commonly caught in the Mediterranean Sea. Its common names include European spiny lobster, crayfish or cray (in Ireland), common spiny lobster, Mediterranean lobster and red lobster..

This species is called in Italian: 'aragosta [mediterranea]' 

#ordering#
In conclusion, as I said, Italian has different words whereas English has different qualifier:  **astice**, omaro = true lobster, *claw* lobster, **aragosta**, palinuro = *spiny* lobster, rock lobster.  "aragosta" has no claws, the English term is more *descriptive*

![enter image description here][1]![enter image description here][2] no claws :  ![enter image description here][3]


If you are treating a friend in Boston you probably have no chance of getting *your European* aragosta, but you can order something quite similar,  a  *Palinurus interruptus*, that is a: [**California spiny lobster**](https://en.wikipedia.org/wiki/California_spiny_lobster).

*buonappetito!*

<sup>*Note: Italian also has an individual word for [*Scyllarus arctus*](http://en.wikipedia.org/wiki/Scyllarus_arctus): cicala di mare = slipper/ locust lobster*</sup>


  [1]: https://i.sstatic.net/4wgXA.png
  [2]: https://i.sstatic.net/vrmCZ.png
  [3]: https://i.sstatic.net/zyCD7.png
  [4]: https://i.sstatic.net/RqIM4.jpg