por see lahs MOHS-kahs
Literally, this means preparing 'for if the flies' show up.
In practice, it means taking a precaution 'just in case'.
Llevaré un paraguas, por si las moscas.
B1I'll take an umbrella, just in case.
Aunque el pronóstico es bueno, coge una chaqueta, por si las moscas.
B1Even though the forecast is good, grab a jacket, just to be on the safe side.
Guarda mi número de teléfono en tu móvil, por si las moscas.
B2Save my phone number in your cell, just in case.
This expression has a very practical and visual origin. It comes from the age-old habit of covering food to protect it from flies. Even if you didn't see any flies at the moment, you would cover the food anyway as a precaution, 'for if the flies' were to appear. This simple, sensible act of preventing a common nuisance evolved into a general expression for taking any precaution against a potential problem, no matter how small.
You can use 'Por si las moscas' all by itself. If someone asks why you're bringing a jacket on a warm day, you can just shrug and say, 'Por si las moscas.' It perfectly communicates 'just in case' without needing any other words.
This phrase is perfect for any situation where you're doing a little something extra to be prepared. Taking extra cash, packing a snack, saving a document twice—all are great moments to use 'por si las moscas'.
Mistake: “Thinking the phrase can only be used when there's a risk of insects or something dirty.”
Correction: The flies are completely metaphorical now. The expression has nothing to do with actual insects and is used for any kind of general precaution, from weather changes to running out of money.
Want to understand the grammar behind this idiom? Explore these lessons for a deep dive:
The idiom starts with 'Por', which is used here to express the reason or motive behind the action (taking precautions), like when you say 'Llevaré un paraguas, por si las moscas'.
This phrase often follows an instruction or command, clearly shown in the examples using verbs like 'coge' (grab) and 'Guarda' (save) to tell someone to take action.
Extremely common. It's a cornerstone of everyday, informal Spanish and is used by everyone.
Widely understood and used in most countries, especially in the Southern Cone (Argentina, Uruguay, Chile). However, the phrase 'por si acaso' is often a more common equivalent in many regions.
Question 1 of 1
If you grab an extra bottle of water for a hike 'por si las moscas', what are you doing?
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Yes, they mean the exact same thing: 'just in case'. 'Por si las moscas' is a bit more colloquial and colorful, while 'por si acaso' is a more standard and direct equivalent. You can use them interchangeably in most informal situations.
